Download Trophic effects through herbivory at Ningaloo Reef

Edith Cowan University
Centre for Marine Ecosystems Research
Trophic effects through herbivory at
Ningaloo Reef
Final report for WAMSI Node 3.2: Biodiversity
assessment, ecosystem impacts of human usage
Adriana Vergès, Glenn Hyndes and Mathew Vanderklift
CMER 2011-01
cmer.ecu.edu.au
Relevant research for better management
Trophic effects through herbivory at Ningaloo Reef
2011
This report has been prepared to summarise the findings and management implications
of WAMSI Node 3: Project 3.2: Biodiversity assessment, ecosystem impacts of human
usage and management strategy evaluation: Trophic effects through herbivory at
Ningaloo Reef.
No portion of this material may be reproduced or communicated without the permission of
ECU, unless the reproduction or communication is authorised by law.
 ECU 2011.
Trophic effects through herbivory at Ningaloo Reef.
Final report for WAMSI Node 3.2: Biodiversity assessment, ecosystem impacts of human
usage and management evaluation.
1
1
1,2
Adriana Vergès , Glenn Hyndes and Mathew Vanderklift ,
1
Centre for Marine Ecosystems Research
School of Natural Sciences
Edith Cowan University
270 Joondalup Drive
Joondalup, WA, 2027
Australia
2
CSIRO Marine and Atmospheric Research
Private Bag 5
Wembley, WA, 6913
Australia
Cite as:
Vergès, A, Hyndes, G.A, Vanderkilft, M.A. (2011). Trophic effects through herbivory at
Ningaloo Reef. Final report for WAMSI Node 3.2: Biodiversity assessment, ecosystem
impacts of human usage and management evaluation. Centre for Marine Ecosystems
Research, Edith Cowan University, Joondalup, Western Australia.
Trophic effects through herbivory at Ningaloo Reef
2011
Table of Contents
Authors............................................................................................................. 5
Acknowledgements .......................................................................................... 6
Executive Summary ......................................................................................... 7
CHAPTER 1. Background ..................................................................... 9
Ningaloo Marine Park and WAMSI .................................................................. 9
Trophic effects in coral-reef systems ............................................................... 9
Need............................................................................................................... 10
Report structure ............................................................................................. 11
CHAPTER 2. Spatial patterns in herbivory on a coral reef are
influenced by structural complexity but not by algal traits..................... 12
Introduction .................................................................................................... 12
Materials and Methods................................................................................... 14
Study area.........................................................................................................14
Patterns in rates of consumption of macroalgae among habitats ............14
Patterns in biomass and species composition of herbivorous fish
among habitats.................................................................................................15
Patterns in cover, biomass and species composition of
macroalgae among habitats ...........................................................................15
Patterns in coral cover and structural complexity .......................................15
Experimental test of effects of habitat and herbivory on algal
consumption, productivity and chemical composition ................................16
Experimental test of palatability of algae from high and low
herbivory habitats ............................................................................................16
Effect of proximity to reef on consumption of macroalgae .........................17
Statistical analyses ..........................................................................................17
Results ........................................................................................................... 18
Seascape patterns in the distribution of herbivory, herbivores,
macroalgae, coral cover and rugosity ...........................................................18
Relationships between macroalgal herbivory and other variables ...........19
Experimental test of effects of habitat and herbivory on algal
consumption, productivity and chemical composition ................................20
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Trophic effects through herbivory at Ningaloo Reef
2011
Experimental test of palatability of algae from high and low
herbivory habitats ............................................................................................. 20
Effect of proximity to reef on consumption of macroalgae ......................... 20
Discussion.......................................................................................................37
Seascape patterns in the distribution of macroalgae, herbivores
and herbivory .................................................................................................... 37
Relationship between patterns in herbivory and macroalgal
distribution, productivity, nutritional quality and palatability ........................ 37
Feedback mechanisms between herbivory and algae ................................ 38
Herbivory patterns explained by structural complexity................................ 39
CHAPTER 3. Herbivore diversity on coral reefs: a transcontinental
comparison........................................................................................... 40
Introduction .....................................................................................................40
Materials and Methods....................................................................................41
Study locations.................................................................................................. 41
Macroalgal assays and video analysis .......................................................... 41
Video analysis ................................................................................................... 42
Relationship between bite rates and macroalgal removal .......................... 42
Distribution of herbivorous fishes ................................................................... 42
Results ............................................................................................................43
Video analysis ................................................................................................... 43
Relationship between bite rates and macroalgal removal .......................... 43
Distribution of herbivorous fishes ................................................................... 44
Discussion.......................................................................................................53
CHAPTER 4. Rates of Sargassum consumption and identity of
herbivores vary across hundreds of kilometres along a continental
fringing coral reef.................................................................................. 55
Introduction .....................................................................................................55
Materials and methods....................................................................................56
Study locations.................................................................................................. 56
Macroalgal tethers ............................................................................................ 56
Video analysis: species-specific bite rates ................................................... 57
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Trophic effects through herbivory at Ningaloo Reef
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Distribution of roving herbivorous fishes ......................................................58
Statistical analyses ..........................................................................................58
Results ........................................................................................................ 59
Removal of transplanted macroalgae ...........................................................59
Species-specific bite rates ..............................................................................60
Feeding behaviour ...........................................................................................60
Relationship between herbivore distribution and macroalgal
removal..............................................................................................................60
Discussion ................................................................................................... 73
CHAPTER 5. Variability in the food sources of herbivorous
invertebrates and fishes in a coral-reef system: a stable isotope
approach............................................................................................ 76
Introduction.................................................................................................. 76
Materials and methods ................................................................................ 77
Study area and sample collection .................................................................77
Stable isotope analysis ...................................................................................78
Data analyses................................................................................................... 78
Results ........................................................................................................ 79
Species and regional comparisons ...............................................................79
Species and habitat comparisons .................................................................80
Discussion ................................................................................................... 93
Variability among roving herbivores ..............................................................93
Spatial variability in the importance of food sources to herbivores ..........94
CHAPTER 6. The role of herbivory on the spatial distribution of
recruiting and established algal communities in coral versus algal
dominated habitats ............................................................................ 96
Introduction.................................................................................................. 96
Materials and Methods ................................................................................ 97
Study location ................................................................................................... 97
Fish community characterisation ...................................................................97
Benthic community characterisation .............................................................98
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Trophic effects through herbivory at Ningaloo Reef
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Algal recruitment experimental design .................................................................. 98
Data analyses ........................................................................................................... 99
Results ..........................................................................................................100
Roving herbivorous fish biomass and community composition ............... 100
Benthic algal biomass and community composition .................................. 100
Benthic community cover .............................................................................. 100
Recruitment tile algal community composition and biomass ................... 100
Discussion.....................................................................................................114
CHAPTER 7. Main findings and outcomes ........................................ 116
References ......................................................................................... 119
Publications and Presentations from this Research........................... 129
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Trophic effects through herbivory at Ningaloo Reef
2011
Authors
David Abecasis - Centro de Ciências do Mar da Universidade do Algarve, Campus de
Gambelas 8005-139 Faro, Portugal.
David Bellwood - School of Marine and Tropical Biology. James Cook University.
Townsville QLD 4811 Australia. Current address: Australian Research Council Centre of
Excellence for Coral Reef Studies, and School of Marine and Tropical Biology. James
Cook University. Townsville QLD 4811 Australia
Scott Bennett - School of Plant Biology. University of Western Australia. Crawley WA
6009 Australia. Current address: Australian Research Council Centre of Excellence for
Coral Reef Studies, and School of Marine and Tropical Biology. James Cook University.
Townsville QLD 4811 Australia
Chris Doropoulos - School of Biological Sciences and ARC Centre of Excellence for
Coral Reef Studies, The University of Queensland, Brisbane, 4072, Australia
Glenn Hyndes - Centre for Marine Ecosystems Research, School of Natural Sciences,
Edith Cowan University, 270 Joondalup Drive, Joondalup, WA, 6027, Australia
Peter Micheal - Centre for Marine Ecosystems Research, School of Natural Sciences,
Edith Cowan University, 270 Joondalup Drive, Joondalup, WA, 6027, Australia
Mathew Vanderklift - Centre for Marine Ecosystems Research. School of Natural
Sciences. Edith Cowan University. 270 Joondalup Drive, Joondalup, WA, 6027,
Australia. Current address: CSIRO Marine and Atmospheric Research, Private Bag 5,
Wembley, WA, 6913, Australia
Adriana Vergés - Centre for Marine Ecosystems Research. School of Natural Sciences.
Edith Cowan University. 270 Joondalup Drive, Joondalup, WA, 6027, Australia. Current
address: Sydney Institute of Marine Science and Evolution and Ecology Research
Centre. School of Biological, Earth and Environmental Sciences. University of New
South Wales. Sydney NSW 2052 Australia
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Trophic effects through herbivory at Ningaloo Reef
2011
Acknowledgements
We are grateful to the team of staff and volunteers for their tireless support in the field and/or
laboratory: A. Bostock, E. Crochelet, R. Czarnik, F. Dalaine, J. Eyres, M. Haywood, K.
Inostroza, P. Kiss, R. Larsen, A. Lemmon, I. McLeod, P. Michael, N. Millar, T. Minutoli, V.
Mocellin, A. Richards, J. Santana, D. Thomson, F. Vitelli and co-authors. We thank J. Tanner
for the stable isotope analyses, and J. Ortiz for statistical advice and thoughtful discussions
on aspects of the study. We also thank the co-authors of the chapters within this report for
their discussions and other contributions: D. Abecasis, D. Bellwood, S. Bennett and P.
Michael. Additional funding through ECU and D. Bellwood at James Cook University allowed
the cross continental comparison in Chapter 3. Insightful comments and discussions were
provided throughout the study by R. Babcock. The Department of the Environment and
Conservation of Western Australia provided a permit to the authors during 2008 -2009 to
perform this study within the Sanctuary Zones of the Ningaloo Marine Park (Permit Number
CE002084) and to collect flora samples (Permit Number SF006457). The study was funded
mainly by the Western Australia Marine Science Institute under the Ningaloo Research
Program.
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Executive Summary
The Ningaloo Marine Park (NMP) is a fringing coral-reef system that encompasses
approximately 263,343 ha along the Pilbarra coast of Western Australia. It was gazetted in
1987 and a revised Management Plan was approved in 2005. The management plan
identified the diversity of fish found within the Ningaloo Marine Park as a key ecological
value, and fishing (particularly recreational) as a major pressure on this value. As a
consequence, the management plan established an objective to “ensure the species
distribution and abundance of finfish species are not unacceptably impacted by recreational
and commercial fishing”. A spatial management strategy has been established using range
of management zones, including sanctuary zones, to achieve this objective. To inform
management of the park, a large research programme was established within the Western
Australian Marine Science Institute (WAMSI). The current study was nested in “Project 3.2:
Biodiversity assessment, ecosystem impacts of human usage and management strategy
evaluation” of WAMSI Node 3.
Coral reefs are diverse ecosystems which host abundant populations of consumers and
algae in spatial mosaics of both coral and algal dominated habitats. A key ecological process
in coral-reef ecosystems is herbivory, which has direct effects on macroalgae and indirect
effects on corals by influencing the outcome of coral-algal competition. Despite high diversity
and abundance of nominally herbivorous fish, recent studies indicate that only a small subset
of taxa are capable of removing dominant macroalgae once these become established. This
limited functional redundancy highlights the potential vulnerability of coral reefs to
disturbance and stresses the need to assess the functional role of individual species of
herbivores.
This study was nested in sub-project 3.2.2 “Ecosystem impacts of human usage and the
effectiveness of zoning for the biodiversity conservation” in Node 3.2. The focus of this study
was on trophic effects in the NMP. Due to the importance of herbivores in coral-reef
systems, this study focused on characterising and quantifying the process of herbivory in the
NMP with a particular emphasis on the removal of adult macroalgae.
We used a range of approaches to gain an understanding of spatial and species-related
patterns in herbivory in five distinct studies. Using underwater video cameras and
Sargassum myriocystum assays, 23 different fish species were observed consuming
macroalgae, but seven species (Naso unicornis, Kyphosus sp., K. vaigiensis, Siganus
doliatus, Scarus ghobban, S. schlegeli and initial-phase Scarus sp.) together accounted for
95% of the observed bites across five regions. Of these species, three were identified as the
most important in consuming macroalgae: N. unicornis, Kyphosus sp. and K. vaigiensis.
These results were supported by stable isotope analyses that incorporate nutrients from food
sources over far longer periods than those examined using the assay approach.
We quantified spatial patterns of macroalgal consumption and food sources across a range
of scales. Firstly, across reef habitats separated by hundreds of meters, herbivory was
always greatest in the structurally complex coral-dominated outer reef and reef flat habitats,
which are also characterised by the highest biomass of herbivorous fish. Secondly, we
showed a high degree of variability in grazing rates among regions separated by 100s km in
the marine park, with different species responsible for macroalgal removal among those
regions. Either N. unicornis or Kyphosus spp. were responsible for the majority of the
grazing. Thirdly, we showed variability in the importance of different food sources across
both habitats and regions for some consumers (e.g. Siganus spp.) but consistency for other
species (e.g. Naso unicornis, Kyphosus spp.), which is likely to reflect shifts in food source
availability or feeding preferences. Lastly, we found strong transcontinental differences
between Keppel Islands in the Great Barrier Reef (GBR) on the east coast of Australia and
Ningaloo Reef in both the diversity of the species observed feeding and on the species
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2011
composition of the roving herbivorous fish community. In Ningaloo Reef, 23 species were
observed biting on Sargassum, compared with just 8 in the Keppel Islands. Sargassum
consumption in the Keppel Islands was dominated by a small number of species and
supports the identification of Naso unicornis as a key browser species. The larger number of
species feeding on macroalgae in Ningaloo Reef suggests that there may be higher
functional redundancy among macroalgal consumers in this system.
We also characterised the benthic community dynamics of the reef-flat and lagoon habitats
to identify seasonal patterns and we experimentally determined the importance of herbivory
on algae recruitment in these two habitats. Differences among habitats in algal biomass
were strongly influenced by season. Lagoon habitats only had higher biomass than reef-flat
habitats during part of the year (late summer/ early autumn). Herbivory had an equally strong
effect on the community composition of algal recruits in the lagoon and reef flat habitats,
despite the reef flat hosting a herbivorous fish community that was an order of magnitude
greater in terms of biomass than the lagoon, which is characterised by younger and smaller
fish (e.g. Scarus initial phase).
Management implications and recommendations are:
•
•
•
•
•
•
•
•
•
•
We have provided a baseline survey of all nominally herbivorous fish species (fish
density and quantitative feeding activity data) across a range of regions and we have
identified key species that should be closely monitored.
We provide evidence that structural complexity is a key factor influencing herbivory.
Conservation efforts should thus focus on conserving this trait (e.g. protect coral
habitats from anchoring damage).
Although herbivorous fishes are not presently targeted by fishers in NMP, this trophic
group is increasingly being targeted for exploitation elsewhere. We have provided
quantitative data that can be used to support potential management plans aimed at
protecting herbivorous fishes from disturbances on the basis of their critical role for
promoting coral-reef resilience.
A direct comparison of herbivory between different coral-reef systems indicates that
Ningaloo Reef is a comparatively pristine system.
Variability in grazing rates across NMP, and the species responsible for grazing,
indicates that any future monitoring of key species needs to take place over different
regions of the marine park.
Herbivory is a dominant mechanism that influences the abundance of fleshy
macroalgae when recruitment space is equal in the lagoon and reef-flat habitats. Zoning
needs to account for the movement of key herbivores across habitats when determining
boundaries of management zones.
Monitoring the biomass of Naso unicornis, Kyphosus spp. inside and outside sanctuary
zones will provide crucial information of the potential influence of zoning on macroalgal
removal in the NMP, as well as a region’s ability to recover from disturbances that
enhance macroalgal production.
Quantitative data on rates of herbivory from our studies can be incorporated into broadscale fish density data from other projects to model the effects of disturbances and
changes in management strategies on herbivory, and potential effects to the system as
a whole.
Data on the spatial patterns of movement are needed for all key macroalgal grazers to
ensure that sanctuary zones preserve their abundances.
Research is needed to further investigate the potential for indirect ecological effects and
trophic cascades through the removal of higher order predators (e.g. sharks) in the
NMP.
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Trophic effects through herbivory at Ningaloo Reef
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CHAPTER 1. Background
Ningaloo Marine Park and WAMSI
The Ningaloo Marine Park, encompassing approximately 263,343 ha, was gazetted in 1987
and a revised Management Plan was approved in 2005. The management plan identified the
diversity of fish found within the Ningaloo Marine Park as a key ecological value, and fishing
(particularly recreational) as a major pressure on this value. As a consequence, the
management plan established an objective to “ensure the species distribution and
abundance of finfish species are not unacceptably impacted by recreational and commercial
fishing”. The primary strategy to achieve this objective was to establish a range of
management zones within the marine park, including sanctuary zones where fishing
activities are excluded.
A large research programme was established at Ningaloo Marin Park as part of “Node 3:
Conserving Marine Biodiversity” of the Western Australian Marine Science Institute
(WAMSI). This Node was established to gain a better understanding of issues including the
effect of climate change, fisheries, tourism, coastal development and industry on the marine
biodiversity and social values of the Ningaloo Marine Park. A range of sub-projects were
nested in “Project 3.2: Biodiversity assessment, ecosystem impacts of human usage and
management strategy evaluation” of WAMSI Node 3:
3.2.1
Diversity, abundance and habitat utilisation of sharks and rays;
3.2.2
Ecosystem impacts of human usage and the effectiveness of zoning for the
biodiversity conservation;
3.3.3
Management strategy evaluation; and
3.3.4
Ningaloo Research program start-up project for impacts of human usage,
oceanography and management strategy evaluation.
This study was nested in “3.2.2 Ecosystem impacts of human usage and the effectiveness
of zoning for the biodiversity conservation”. The focus of this study was on the
trophodynamics and trophic effects in the Ningaloo Marine Park. Due to the importance of
herbivores in coral-reef systems (see below) this study focused on gaining an understanding
of the role of herbivores in maintaining the balance between corals and macroalgae in this
coral-reef system.
Trophic effects in coral-reef systems
Herbivory is a key ecological process in coral reefs that is considered fundamental to the
balance between corals and macroalgae. A diverse herbivorous community is thought to
strongly influence the resilience of coral reefs, i.e. their ability to reorganise and maintain
ecosystem function following disturbance (Bellwood et al. 2004; Mumby et al. 2006). Indeed,
overfishing of roving herbivorous fishes is considered a major factor contributing to reef
degradation. This is often linked to phase shifts from coral to macroalgal dominance
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Trophic effects through herbivory at Ningaloo Reef
2011
(Hughes 1994; McClanahan et al. 2001; Graham et al. 2006). It is therefore crucial to
understand the role that roving herbivorous fishes play in maintaining that balance between
macroalgae and coral. In gaining that understanding, it is important to note that roving
herbivorous fishes have recently been shown to exhibit widely varying feeding modes and
diets (Choat et al. 2002; Choat et al. 2004; Crossman et al. 2005), rather than conforming to
a uniform functional group (Bellwood et al. 2004; Green and Bellwood 2009; Hoey and
Bellwood 2010). It is crucial, therefore, to gain an understanding the varying roles of these
herbivores in coral-reef systems for effective management of these systems.
Need
Understanding key ecological processes in marine landscapes is fundamental to effective
marine management, particularly when management utilises spatial strategies such as
marine protected areas (MPAs). MPAs are a management tool used in numerous countries
for a variety of objectives (Halpern 2003). Marine parks form a major management tool for
management of the marine environment by the Department of Environment and
Conservation in Western Australia. Like many marine parks in Western Australia, the
Ningaloo Marine Park was established to maintain biodiversity and ecological integrity
through a multiple-use spatial management system. The establishment of zones designated
for different levels of use allows for the continued use of the area, but provides a higher level
of protection than would otherwise be the case.
As stated above, herbivores play a critical role in the resilience of coral-reef systems, yet
they are exposed to human impacts. Despite their importance, very little is known about the
roles of different types of herbivorous fish and invertebrates in the Ningaloo Marine Park.
The vast majority of work on roving herbivorous fishes has taken place in the Great Barrier
Reef on the east coast of Australia, or in the Caribbean Sea. Thus, there is a need to gain an
understanding of the key species responsible for the removal of macroalgae in the Ningaloo
Marine Park. Together with other WAMSI Node 3.2 studies examining fish distributions
inside and outside sanctuary zones, as well as movement patterns in the marine park, an
understanding of herbivory will provide the basis for an assessment of any shifts in herbivory
associated with changed zoning within the marine park.
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Trophic effects through herbivory at Ningaloo Reef
2011
Report structure
The broad aim of this study was to gain an understanding of the trophic effects of herbivores
and their varying roles in the Ningaloo Marine Park. We also aimed to determine the
variability in the consumption of food sources at different spatial scales throughout the park,
providing critical information for spatial management. The report will provide the results of a
range of studies related to herbivory using multiple descriptive and experimental
approaches. The results of each discrete study are presented as individual chapters, each
providing an introduction, methods, results, discussion and conclusion, as outlined below:
1.
2.
3.
4.
5.
Spatial patterns in herbivory on a coral reef are influenced by structural complexity but
not by algal traits.
Herbivore diversity on coral reefs: a transcontinental comparison.
Variation in macroalgal herbivory by fishes across a western-continental coral-reef
system.
Variability in the food sources of herbivorous invertebrates and fishes in a coral-reef
system: a stable isotope approach.
The role of herbivory on the spatial distribution of recruiting and established algal
communities in coral versus algal dominated habitats.
All but Study 2 focused entirely on herbivory patterns and processes at Ningaloo Reef. We
were provided with an opportunity to collaborate with researchers at James Cook University
to carry out a comparative study (Study 2) on herbivory rates between Ningaloo Reef and
GBR. This allowed us to place herbivory rates in the context of another large coral-reef
system, and provided us with the technical skills to examine the regional patterns in
herbivory at Ningaloo Reef (Study 3).
Finally, the report provides a synthesis of the conclusions from each chapter and discusses
the implications of our findings to marine management in the region.
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Trophic effects through herbivory at Ningaloo Reef
2011
CHAPTER 2. Spatial patterns in herbivory on a coral reef are
influenced by structural complexity but not by algal traits
Adriana Vergés, Mathew A. Vanderklift, Christopher Doropoulos, Glenn A. Hyndes
Introduction
Spatial heterogeneity in ecosystems can strongly influence population structure, community
composition and ecosystem processes (Pickett & Cadenasso 1995). Herbivory can generate
spatial heterogeneity by regulating rates of primary production and nutrient cycling
(Augustine & McNaughton 1998, Augustine & Frank 2001), modifying plant community
composition, diversity and biomass (Milchunas & Lauenroth 1993, Hulme 1996), and/or
directly disturbing habitats physically (e.g. through burrowing; (Huntly & Inouye 1988, Knapp
et al. 1999)). Several factors are known to control the spatial distribution of herbivory,
including abiotic influences such as topography or distance to water (in terrestrial systems),
and biotic influences such as plant distribution, nutritional quality, predation, herbivore social
behaviour (e.g. herding), and human management practices (Coughenour 1991, Bailey et al.
1996, Adler et al. 2001, Ogutu et al. 2010). Additionally, feedback mechanisms between
herbivory and plant quality can also influence spatial patterns of herbivory. For example,
while herbivory generally decreases plant biomass, it often enhances nutrient recycling and
availability (McNaughton et al. 1997, Augustine et al. 2003), although these short-term
positive feedbacks may eventually result in a compositional shift towards less palatable plant
species (Pastor et al. 1993). However, much of this knowledge comes from wildlife and
rangeland management literature that deals mostly with large ungulates, and we know
comparatively less about what controls spatial patterns of herbivory in ecosystems
characterised by other consumers.
This study focuses on the processes that control the spatial distribution of herbivory in coral
reefs, ecosystems that are characterised by some of the highest rates of herbivory (Hay
1991, Hay & Steinberg 1992, Augustine & Frank 2001). Herbivores can remove over 90% of
daily algal production in shallow coral reefs (Hatcher & Larkum 1983, Carpenter 1988,
Augustine & Frank 2001), and the presence of abundant coral depends on high levels of
herbivory (Lewis 1986, Burkepile & Hay 2006, Hughes et al. 2007). Indeed, herbivorous fish
play a crucial role in maintaining coral-reef resilience (i.e. the ability of a system to absorb
disturbances whilst maintaining ecosystem function (Holling 1973)) by consuming
macroalgae that can otherwise outcompete corals when new space becomes available
following disturbance (Hughes 1994, McClanahan et al. 2001, McCook et al. 2001, DiazPulido & McCook 2003, Mumby et al. 2006). However, the impact of herbivores is usually not
uniform across all habitats, and coral reefs may be viewed as spatial mosaics of animal- and
macroalgae-dominated communities characterised by different intensities of herbivory (Hay
1981a, Lewis & Wainwright 1985, Lewis 1986, Fox & Bellwood 2007). Thus, variations in the
intensity of herbivory between different parts of a reef separated by short distances (tens to
hundreds of meters) can be greater than differences among reefs that are many kilometres
apart (Russ 1984). Despite the existence of such marked spatial patterns in herbivory in
coral reefs, we have a limited knowledge of the factors that drive differences across reef
gradients.
Early studies dealing with spatial patterns in coral reefs focused on the distribution of marine
plants, and highlighted the importance of herbivory for maintaining differences among
habitats (Hay 1981a, Hatcher & Larkum 1983, Hay et al. 1983a, Lewis 1986, McCook 1997).
Other studies focused on the distribution of herbivores in different sections of the reef and
found marked variations in densities and species composition (Russ 1984, Lewis &
Wainwright 1985, Fox & Bellwood 2007, Wismer et al. 2009). Several mechanisms involving
12
Trophic effects through herbivory at Ningaloo Reef
2011
both abiotic and biotic influences have been proposed to explain the striking gradients in the
intensity of herbivory observed on coral reefs worldwide. Among the abiotic influences, wave
exposure and depth are considered to inhibit herbivory. Herbivory is often lowest in the first
1-2 meters of water, where turbulence associated with wave impact hinders the feeding
ability of fish, it is greatest a few meters below the surface, and decreases at greater depths
(> 20m) (Hay 1981b, Vergés et al. 2009, Brokovich et al. 2010). In contrast, structural
complexity and availability of refuges are considered to enhance herbivory (Hay 1981b, Hay
et al. 1983a, Lewis & Wainwright 1985, McCook 1997). In terms of biotic factors, large
grazers appear to aggregate in zones of high algal turf production, although the mechanisms
by which fish respond to productivity are not clear (Russ 2003). Despite many such
hypotheses having been invoked to explain spatial variation of herbivory in coral reefs, few
studies have experimentally tested the importance of specific mechanisms. Moreover,
herbivory in coral reefs is a process that involves a wide range of consumers with highly
variable feeding modes and diets and with contrasting ecological functions (Choat 1991),
and there is a need to quantify and assess the impact of different functional groups
separately. For example, differences in turf algae productivity may influence (and be
influenced by) the distribution of fishes that consume turf algae (Russ 2003), but probably
have no effect on species that feed on macroalgae.
Roving herbivorous fishes have been clearly identified as the key herbivores in undisturbed
coral reefs (Hughes et al. 2007). However, they do not constitute an ecologically uniform
group, but can be broadly classified into grazer and browser functional groups, depending on
their diet and mode of feeding (Bellwood et al. 2004, Hoey & Bellwood 2010a). Grazing taxa
(including scraping and excavating parrotfishes) typically feed on the epilithic algal matrix
(EAM sensu Wilson (Wilson et al. 2003)) and on crustose coralline algae, and constitute the
majority of herbivorous fishes on coral reefs. In contrast, only a handful of species are
considered to be browsers – that is, species that consume large erect macroalgae (Bellwood
et al. 2006a, Fox & Bellwood 2008, Hoey & Bellwood 2009, Bennett & Bellwood 2011).
Grazers and browsers are thought to play distinct and complementary roles in avoiding
phase shifts towards macroalgal dominance (Bellwood et al. 2004, Bellwood et al. 2006a).
Grazers can preclude an increase in overall algal biomass, prevent macroalgal growth by
consuming macroalgal recruits, and provide space for coral recruitment, while browsers
consume the adult brown seaweeds that typically dominate coral reefs in the absence of
herbivory, and therefore have the potential to reverse phase shifts once macroalgae are
established in reefs (Bellwood et al. 2004, Bellwood et al. 2006a). Recent studies have
highlighted the importance of macroalgal consumption and have identified the key species or
functional groups responsible for this ecological function (Bellwood et al. 2006a, Fox &
Bellwood 2007, Hoey & Bellwood 2009, Cheal et al. 2010, Bennett & Bellwood 2011), but we
know little about the mechanisms that control the distribution and abundance of these
browsers.
In this study, we quantified spatial patterns of macroalgal herbivory by fishes across a coral
reef, tested for similar spatial patterns in potential explanatory variables, and then used
manipulative and mensurative experiments to test some hypotheses arising from the
patterns observed. We tested for the presence of spatial variation in herbivory by quantifying
consumption of erect macroalgae and measuring the biomass and composition of
herbivorous fishes among a cross section of a coral reef (lagoon, reef flat and outer reef
habitats). We then related patterns in herbivory to the spatial distribution of algal cover, algal
biomass and structural complexity in these three habitats. In the manipulative and
mensurative experiments, we selected two habitats with contrasting levels of herbivory (reef
flat and lagoon) and used herbivore exclusion and feeding experiments to test hypotheses
about the mechanisms that might cause the observed consumption patterns. In particular,
we asked: (1) Does consumption of macroalgae relate to spatial patterns in macroalgal
productivity, nutritional quality, community composition and/or palatability? (2) Does
herbivory influence macroalgal nutritional quality? and (3) Does benthic structural complexity
and proximity to reef influence the distribution of macroalgal herbivory.?
13
Trophic effects through herbivory at Ningaloo Reef
2011
Materials and Methods
Study area
This study was conducted at Ningaloo Reef (Western Australia), a fringing reef
approximately 290 km in length. Ningaloo Reef is an arid-zone reef where extensive coral
reefs occurs in close proximity to the mainland. The study was conducted between April
2008 and September 2009 in the Mandu (22° 06’ S, 113° 52’ E) and Maud (23° 07’ S, 113°
44’ E) sanctuary zones of the Ningaloo Marine Park. At Ningaloo, the reef crest is narrow
and mostly devoid of coral growth, the reef flat landward of the reef crest hosts coral
communities across a width of approximately 150 m, and the outer reef slopes seaward of
the reef crest presenting a well-developed spur and groove morphology to depths of 30 m
(Cassata & Collins 2008) (Digital Appendix Fig 1). In the Mandu sanctuary zone, the reef
encloses a lagoon that is about 1 km in width. In the Maud sanctuary zone, the width of the
lagoon ranges more widely from 1.7 km to 7 km. In each location, the lagoon is populated
with sparse corals, sandy substrata and patches of macroalgae. The tides in the area have a
maximum ~ 2 m range at spring tides.
Most of this study took place in the Mandu sanctuary zone, where we haphazardly selected
three study sites in each of three habitats that characterise the area: lagoon, reef flat and
outer reef (Digital Appendix Fig. 1). We performed one experiment (on the effects of reef
proximity on macroalgal removal) in the Maud sanctuary zone, where we haphazardly
selected three study sites at the boundary between the reef flat and lagoon, with each site
located about 250 m apart.
Patterns in rates of consumption of macroalgae among habitats
Relative differences in consumption of erect macroalgae between lagoon, reef flat and outer
reef habitats at all Mandu sites were measured in April 2008 using tethered pieces of
Sargassum myriocystum. This brown alga is rapidly eaten (within hours) and has a relatively
simple morphology, which allowed us to estimate biomass loss from differences in length
before and after deployment. S. myriocystum plants bear 3 – 6 main lateral branches per
individual, and each of these has a consistent length-weight relationship (148.01 + 9.241
mg/cm). Patterns of herbivory were determined by placing lateral branches of S.
myriocystum of 15 cm in length and similar weight (mean 2.13 + 0.19 g; n = 26) in three sites
at each of the lagoon, reef flat and outer reef habitats. In each site, S. myriocystum lateral
branches (n=25) were distributed haphazardly and tethered to the available substrata using
cable ties. In the lagoon habitat, branches were either tethered to other macroalgae or to
lose pieces of dead coral on the sand at around 1.5 m depth. In the reef flat habitat,
branches were mostly tethered to pieces of dead coral covered in epilithic algal matrix
adjacent to live corals, at 1-2 m depth. In the outer reef habitat, branches were mostly
tethered to coral pieces and crustose coralline rocky surfaces at about 6 m depth – the
shallowest depth that we could easily access in regular swell conditions. Tethered algae
were collected 4-7 hours after deployment, and mass consumed was estimated from the
total length consumed, and converted to mass consumed per hour. Some tethered algae
became detached and lost, leading to an unbalanced data set (final n ranged from 17 to 25
depending on site). Replicates where algae became wholly detached were not included in
the analysis because we could not be sure that detachment was due to herbivory.
14
Trophic effects through herbivory at Ningaloo Reef
2011
Patterns in biomass and species composition of herbivorous fish among habitats
Censuses of the herbivorous fish assemblage were carried out during a two-week period in
November 2008 at three sites in each of the three habitats in the Mandu Sanctuary zone (Fig
2.1). Fishes were counted along eight 25 x 5 m haphazardly placed transects per site during
daylight hours (avoiding 2 hours after sunrise and before sunset). Fish counts were
performed swimming at a constant speed (ca. 8 minutes per 25 m transect) and counting
and estimating the size of fish within 2.5 m of either side of the transect line. Fishes were
identified to species level and their total length was estimated in 5 cm size categories. Size
estimates were validated using objects of known length. Length estimates for individual fish
b
were converted to biomass using the allometric length-weight conversion W= a * TL , where
W is weight in grams, TL is total length and parameters a and b are constants obtained from
the literature (Froese & Pauly 2005). We restricted our counts to mobile herbivorous and
‘nominally’ herbivorous fishes, excluding pomacentrids (Choat et al. 2002). We identified 25
species from the families Acanthuridae, Siganidae, Kyphosidae and Labridae (parrotfishes).
These data were analysed in two ways: one including all species of roving herbivorous fish
and one including only species that are considered to be browsers (consumers of
macroalgae). Of the species recorded, 11 taxa were identified as browsers of erect
macroalgae based on gut content analyses (Westera 2003) and direct observations on
remote video cameras (unpubl. data): Kyphosus vaigiensis, Naso lituratus, Naso spp., Naso
unicornis, Scarus ghobban, S. schlegeli, initial-phase Scarus sp., Siganus argenteus, S.
doliatus, and S. trispilos.
Patterns in cover, biomass and species composition of macroalgae among habitats
To determine whether spatial patterns in macroalgal herbivory were related to macroalgal
distribution, we measured algal cover and algal biomass and community composition at each
site at Mandu in November 2008. Algal cover was quantified using the line intercept benthic
survey method described by Fox and Bellwood (Fox & Bellwood 2007). We conducted a total
of 6 replicate transects (total of 30 points per replicate) in each of the habitats at the three
sites.
2
Macroalgal biomass and community composition were measured by clearing three 0.25 m
haphazardly placed quadrats of all macroalgae (arbitrarily defined as algae with thallus
larger than 1 cm) at each site. Algal samples were bagged and returned to the laboratory,
where they were sorted to genus level (where possible) and weighed. Algal taxa that we
were unable to identify were classified according to broad functional groups (brown, green or
red; filamentous, encrusting or foliose).
Patterns in coral cover and structural complexity
To determine whether patterns in consumption of macroalgae were related to topographic
complexity, we measured coral cover (which provides three-dimensional structure and
potential refuges) and estimated a rugosity ratio (n = 3) at each site. Live coral cover was
quantified using the line intercept benthic survey method described above for algal cover. To
measure rugosity, a 10 m light chain was placed along the substrate contour, and the
equivalent straight line horizontal distance encompassed by the 10 m of chain was
measured (n = 3). The rugosity ratio (R) was calculated as the straight line horizontal
distance along the reef divided by the total chain length, with values close to unity indicating
a flat substratum and lower values indicating a structured habitat (McClanahan & Shafir
1990).
15
Trophic effects through herbivory at Ningaloo Reef
2011
Experimental test of effects of habitat and herbivory on algal consumption, productivity and
chemical composition
A transplant experiment was set up to determine the effects of habitat on the consumption
and growth of algae and to assess the interactive effects of habitat and herbivory on algal
chemical traits. The experiment took place over 6 weeks from April to May 2008 in the
Mandu sanctuary zone. Specimens of Lobophora variegata (ruffled morphotype, sensu Coen
and Tanner (Coen & Tanner 1989)) were randomly collected from a lagoon location and
placed on reef flat and lagoon habitats under caged and uncaged conditions. This species
was chosen because it is commonly found in all coral-reef habitats and because preliminary
feeding trials showed it was consumed at a lower rate than other macroalgae (unpubl. data),
thus making it more suitable for long-term transplant experiments than other species that are
consumed within hours when placed in the reef flat. Three L. variegata thalli were placed
within each plot. All plots were randomly distributed and placed about 3-5 m apart from each
other.
2
We used triangular cages of 1082 cm (equilateral triangle with sides of 50 cm and 50 cm in
height). Open (uncaged) plots were marked with steel reinforcing bar along the corners. In
caged plots, fences and roofs were made with plastic-coated metal mesh (2.5 cm mesh
size), thereby excluding large herbivorous fish. Partial cages consisted of steel reinforcing
bar marks along the corners with one fenced side and a roof, and were used to control for
cage artefacts. All plots had a base of plastic coated mesh to which thalli were attached. The
experiment ran for 6 weeks, and the cages were cleaned of fouling organisms once after 2
weeks, although these were not abundant. We recorded blotted wet-weight of algae (n = 3)
at the beginning and at the end of the experiment and calculated biomass change.
At the end of the experiment, algal thalli were freeze-dried and ground. Nitrogen and carbon
content of individual thalli were analysed using a Europa Scientific ANCA-NT 20-20 mass
spectrometer. Where possible, L. variegata sections of intermediate age (i.e. equidistant
from holdfast and thallus edge) were used in the chemical analyses, and care was taken to
gently remove any epibiota. Some thalli were so heavily grazed that we used the entire
thallus in the analysis, and in some instances we did not have enough mass to conduct
chemical analyses, leading to an unbalanced data set.
Experimental test of palatability of algae from high and low herbivory habitats
We compared the palatability and chemical composition of L. variegata from high and low
herbivory habitats (reef flat and lagoon, respectively) with an experiment performed in April
2008 in the Mandu sanctuary zone. Lagoon and reef flat habitats are characterised by
hosting different morphotypes of L. variegata. As in other coral-reef ecosystems, an erect
ruffled form (hereafter referred to as ‘lagoon’ morphotype) is usually found on sandy
substrata where herbivores are less abundant, whereas the flat decumbent form (hereafter
referred to as ‘reef flat’ morphotype) is usually found underneath coral plates in coraldominated habitat where herbivores are often more common (Coen & Tanner 1989). The
lagoon and reef flat morphotypes of L. variegata were offered in pairs of similar initial area in
the reef-flat habitat, and replicate pairs (n = 15) were at least 3 metres apart from each other.
An equal number of controls (n = 15) were individually protected from herbivores with plastic
window-screen mesh cages (3 mm mesh size). L. variegata pairs were left in the field for five
days. Replicates with one or two wholly detached algae were not included in the analysis
because we could not guarantee that detachment was due to herbivory. Photographs of
each algal specimen were taken at the beginning and at the end of the experiment and
consumption was measured as changes in area determined using ImageJ analysis software.
Five additional thalli of each morphotype were collected at the beginning of the experiment
for carbon, nitrogen and phenolic chemical analyses to further identify potential differences in
16
Trophic effects through herbivory at Ningaloo Reef
2011
nutritional traits between habitats. Total phenolic content was quantified
spectrophotometrically using a modified Folin-Ciocalteu assay (Bolser et al. 1998).
Effect of proximity to reef on consumption of macroalgae
To test the effects of proximity to reef on macroalgal removal, we tethered Sargassum
myriocystum lateral branches at a range of distances from the reef flat/ lagoon boundary and
measured the amount of algae consumed after 48 hours. We predicted that if structural
complexity positively influences herbivory, the rates of macroalgal removal would be higher
near the structurally complex reef flat habitat than in flat lagoon habitat. This experiment was
performed in the Maud sanctuary zone. We were unable to perform this experiment in the
Mandu sanctuary zone because the reef flat/lagoon boundary in that part of the Ningaloo
reef is diffuse, with isolated coral heads scattered irregularly near the boundary. In contrast,
the reef flat and lagoon habitats are clearly defined at Maud. These two sanctuary zones
have similar rates of macroalgal removal and a similar herbivorous fish assemblage (Michael
et al., unpublished data). Sargassum myriocystum (n = 3) lateral branches of about 40 cm in
length were tethered at 9 distances relative to the reef flat/ lagoon boundary (-25, 0, 5, 10,
15, 25, 30, 50 and 75 m) at each of three separate sites. Replicates within each distance per
site were approximately 15 m apart and parallel to the reef flat/lagoon boundary, and sites
were approximately 250 m apart. The length of each individual S. myriocystum lateral branch
was measured at the beginning and at the end of the experimental period. Since sand was
the most common substrate away from the reef, S. myriocystum lateral branches were
tethered to lose pieces of dead coral that were buried in the sand.
Statistical analyses
All data were checked for normality and equality of variances by visual inspection of
scatterplots and distribution of residuals (Quinn & Keough 2002). Where appropriate, data
were transformed to conform to parametric assumptions. When assumptions of normality
could not be met, the significance of effects was assessed by permutation (McArdle &
Anderson 2001).
Patterns in consumption of tethered algae, total fish biomass, coral cover and in the species
composition of herbivorous fish and algae communities were analysed using PERMANOVA
testing for differences between sites (3 levels, random factor) and habitats (3 levels, fixed
factor). Patterns in the cover and biomass of macroalgae and in rugosity were analysed
using analysis of variance (ANOVA) with the same design. When significant differences were
detected between main effects in ANOVA, Student-Newman-Keuls (SNK) tests were used to
resolve the differences among means.
Bray-Curtis distance was our metric in all multivariate analyses and data were fourth-root
transformed prior to analyses to reduce the effects of numerically large values (i.e. abundant
schooling species) (Clarke 1993). Multivariate differences of fish and algae communities
between sites and habitats were visualised using CAP. This procedure produces a
constrained ordination and presents data on the axes chosen to best distinguish groups in
the data (Anderson & Willis 2003). CAP also provides misclassification errors by carrying out
a leave-one-out allocation of observations to groups (habitats), thus indicating the
robustness of the classification. In addition, species with the highest contribution to
differences among habitats were identified as those that had the highest absolute Pearson
correlation with the canonical axis from the CAP analysis. A correlation of r > 0.4 was used
as an arbitrary cut-off to display potential relationships between individual species and the
canonical axes.
We used regressions to determine whether rates of herbivory matched patterns in algal
cover, algal biomass and rugosity, and to assess the relationship between herbivory rates
and roving herbivorous and browser fish biomass. All data were averaged at the site level.
17
Trophic effects through herbivory at Ningaloo Reef
2011
Linear, polynomial and logarithmic regressions were fitted to the data, and the significant
regression that best-fit the data was selected using Akaike’s Information Criterion.
In the herbivore exclusion x habitat experiment, changes in biomass and chemical
composition in L. variegata after 6 weeks were analysed using a four-way ANOVA, testing
for differences among sites (3 levels, random), habitats (2 levels, fixed), large herbivoreexclusion treatments (termed ‘herbivory effect’ throughout; 3 levels, fixed) and plots (3 levels,
random and nested in the interaction of Site x Habitat x Herbivory). Differences in feeding
between lagoon and reef flat L. variegata were analysed using a t-test as outlined by
Peterson and Renaud (Peterson & Renaud 1989) to adequately incorporate controls for
mass changes not due to consumption. The t-statistic was calculated by comparing the
between-food differences in loss of mass of treatments (Choice 1 – Choice 2, with
herbivores) with the between-food differences in loss of mass of control replicates (Choice 1
– Choice 2, without herbivores). Differences in macroalgal chemical traits between
morphotypes were analysed using two-sample t-test when variances were homogenous or
Welch’s t-test otherwise. Confidence intervals (CI) at 95% of t-test results are presented to
assess the validity of non-significant results following Colegrave and Ruxton (Colegrave &
Ruxton 2003). The effects of proximity to reef on macroalgal removal were analysed using a
two-way ANOVA testing for differences among sites (3 levels, random) and distances (9
levels, fixed).
All PERMANOVA and multivariate analyses were performed using Primer-E v6 software
(Clarke & Gorley 2006) with the PERMANOVA+ add-on package (version 1.0.1; (Anderson
et al. 2008a) ). All ANOVAs were performed using the statistical package GMAV5 (coded by
A. J. Underwood and M. G. Chapman, University of Sydney, Australia). T-tests and
regression analyses were performed using R software (Version 2.9.0) (Team 2009).
Results
Seascape patterns in the distribution of herbivory, herbivores, macroalgae, coral cover and
rugosity
There was a significant difference in the rates of herbivory (measured as consumption of
tethered Sargassum) among habitats, a pattern that was constant at all sites (Fig 2.1, Table
2.1). No consumption was recorded in the lagoon, whereas in the reef flat and outer reef
-1
habitats we found similar rates of about 1-2 cm h (permutational analysis of variance
(PERMANOVA) pair-wise tests: Lagoon < Reef flat = Outer reef; p < 0.02). Since Sargassum
was much more abundant in the lagoon than in other habitats (see below), and consumption
may depend on local availability of this resource, we incorporated Sargassum availability into
our analysis of consumption by comparing a relative consumption index (RCI) for this taxa
across habitats, where RCI = proportion of Sargassum consumed x proportion of Sargassum
present (from algae biomass results, data averaged at site level; method modified from GPI
index used by Hoey and Bellwood (Hoey & Bellwood 2009). We found equally striking
differences between habitats in RCI (F2, 6 = 15.89; p = 0.004; data not shown) as for rates of
herbivory.
We found strong differences among habitats in total biomass of all roving herbivorous fish
and of browsing fish alone, a pattern that was consistent at all sites (Table 2.2a, 2.2b). The
reef flat and outer reef habitats generally hosted an order of magnitude higher herbivore
biomass than the lagoon (Fig. 2.2a, 2.2b; PERMANOVA pair-wise tests, p < 0.05 for
comparison between lagoon and either outer reef or reef flat). The species composition of
roving herbivorous fish assemblages was different among habitats but the nature of these
differences varied among sites (significant Habitat x Site interaction; Table 2.3a). Pair-wise
comparisons showed that roving herbivorous fish assemblages in the three habitats were
18
Trophic effects through herbivory at Ningaloo Reef
2011
significantly different from each other at all sites (p ≤ 0.002 for all comparisons), but within
each habitat, assemblages were only similar among sites in the reef flat and outer reef
habitats, and not in the lagoon – i.e. the interaction was caused by the greater degree of
variability in the lagoon. Differences in the composition of the browser fish assemblages
between habitats were less consistent across sites (significant Habitat x Site interaction
yielded by the PERMANOVA analysis: Table 2.3b), with significant differences among all
three habitats at two of the sites (PERMANOVA pair-wise tests, p < 0.05 for all
comparisons), but not at the other site, where only lagoon and reef flat assemblages differed
(PERMANOVA pair-wise test, p = 0.008).
Canonical analysis of principal components (CAP) of all roving herbivorous fish yielded a
high classification success of 93.1% across all habitat types (i.e. only 6.9% misclassification
error). Correlations with CAP axis scores indicate that a high biomass of Chlorurus sordidus
was characteristic of the reef flat (Fig 2.3a), where the biomass of this species was about an
order of magnitude higher than that in the lagoon or outer reef. Although less abundant,
Siganus trispilos was also characteristic of the reef flat habitat. Scarus frenatus, S.
prasiognathos and S. rubroviolaceous characterised the outer reef habitat, with average
biomass for each species in the outer reef 20 times higher than those in the other habitats.
No species were identified as characteristic of the lagoon habitat; this habitat was instead
characterised by a low biomass of all species. CAP of browser fish assemblages yielded a
low classification success of 59.7% (Fig 2.3b). Correlations with CAP axis scores indicated
that the outer reef tended to be characterised by high biomasses of Naso unicornis and N.
lituratus, while the reef flat was characterised by higher biomass of Scarus schlegeli (Fig
2.3b).
There were differences among habitats and sites in algal cover (Fig 2.c; Table 2.2c) and
among habitats in algal biomass (Fig. 2.2d; Table 2.2d). Algal cover and biomass were
higher in the lagoon than in either the reef flat or the outer reef, which were similar (SNK
pair-wise comparisons: Lagoon > Reef flat = Outer reef, p < 0.05). Differences in macroalgal
species assemblages among habitats were not consistent among sites (significant Habitat x
Site interaction; Table 2.3c). Lagoon and outer reef habitats differed significantly at all three
sites (p < 0.05), but comparisons of reef flat habitat with lagoon and outer reef were not
significant anywhere. CAP yielded a classification success of 70.4% across all habitat types
(i.e. 29.6% misclassification error). The reef flat habitat hosted high biomass of Lobophora
variegata, Turbinaria ornata and an unidentified filamentous green alga, whereas the outer
reef habitat was characterised by red algae belonging to the genera Amphiroa and Amansia
(Fig 2.3c). The lagoon habitat was strongly characterised by Sargassum species, which
represented over 80% of the total algal biomass in this habitat.
Coral cover differed among habitats, but the nature of this difference varied among sites
(significant Site x Habitat interaction; Fig 2.2e, Table 2.2e). Coral cover was always lower in
the lagoon than in any other habitat (0 - 5% overall cover; p ≤ 0.002 for all comparisons), but
differences in coral cover between reef flat and outer reef were not consistent between sites.
There were clear differences in rugosity among habitats, a pattern that was consistent at all
sites (Fig 2.2f, Table 2.2f). The lagoon was the least structurally complex habitat, with
rugosity values approaching 1 (mean all sites 0.95 + 0.01; p ≤ 0.01 for all comparisons),
whereas the outer reef had similar rugosity values to the reef flat, which were ~50% more
structurally complex than the lagoon (Fig. 2.2f).
Relationships between macroalgal herbivory and other variables
We found a near-significant logarithmic relationship between site averages for
2
measurements of macroalgal herbivory and algal cover (Fig 2.4a; F1, 7 = 4.958; p = 0.06; r =
2
0.415) but not between macroalgal herbivory and algal biomass (F1, 7 = 3.26; r = 0.318; p =
0.11). In addition, there were no significant relationships between measurements of
2
herbivory rates and total herbivorous fish biomass (F1, 7 = 1.392; r = 0.166; p = 0.277), or
2
browser fish biomass (F1, 7 = 0.822; r = 0.105; p = 0.395). Rates of herbivory were higher in
19
Trophic effects through herbivory at Ningaloo Reef
2011
the sites and habitats that were more structurally complex, as reflected by a strong linear
relationship between rates of consumption of tethered Sargassum and rugosity (Fig 2.4b; F1,
2
7 = 64.82; r = 0.90; p < 0.001), where rugosity explained 90% of the variance in consumption
2
(r = 0.90).
Experimental test of effects of habitat and herbivory on algal consumption, productivity and
chemical composition
Herbivore exclusion experiments performed with Lobophora variegata in high and low
herbivory habitats (reef flat and lagoon, respectively) showed that changes in algae biomass
were strongly influenced by the habitat in which thalli were deployed and whether thalli were
caged, as shown by a Habitat x Herbivory interaction (Fig 2.5a, Table 2.4a). In the reef flat
habitat, caging had an acute effect on biomass change: there was a net increase in biomass
of 30% inside cages, compared to a net decrease in biomass of 30-50% in the partial and
open cages (SNK pairwise comparisons: Caged > Open = Partially Caged; p < 0.01). In
contrast, caging had no effect on algal biomass in the lagoon habitat, where there was a net
increase in biomass of 30-60% in all treatments (SNK pairwise comparisons: Caged = Open
= Partially Caged). The clear inference from this result is that herbivores strongly reduced L.
variegata biomass on the reef flat, but not in the lagoon. No artefacts were associated with
the structure of the cages (SNK pairwise comparisons: Partially Caged Lagoon = Open
Lagoon, and Partially Caged Reef flat = Open Reef flat), i.e. the presence of cages did not
confound the interpretation of the effects of herbivory on algal biomass. In the absence of
herbivory, there were no differences in Lobophora variegata biomass accumulation between
habitats (Fig 2.4a; SNK pairwise comparisons not significant: Reef flat Caged = Lagoon
Caged).
Nitrogen content was significantly higher in caged algae than in open and partial cages
(PERMANOVA pair-wise tests p < 0.05) (Fig 2.5b, Table 2.4b), but the habitat in which algae
were placed had no effect on nitrogen content. In contrast, the availability of nitrogen per unit
carbon (C:N ratio) was not affected by caging but was significantly influenced by the habitat
in which algae were placed, with highest C:N ratios found in algae transplanted to lagoon
habitats (Figure 2.5c, Table 2.4c).
Experimental test of palatability of algae from high and low herbivory habitats
We detected no significant difference in area loss between lagoon and reef flat morphotypes
of Lobophora variegata after 5 days of deployment (Fig 2.6a; t = -0.6359, df = 12, p = 0.537,
95% CI = -4.006, 1.416). The two morphotypes did not differ in their nitrogen content (Fig
2.6b; Welch’s t = 0.170, df = 4.657, p = 0.873; 95% CI = -0.134, 0.153). The C:N ratio of the
reef flat morphotype tended to be lower than the lagoon morphotype, although statistical
differences between the two only approached significance (Fig 6c; t = -2.057, df = 8, p =
0.074). The 95% confidence intervals of this near-significant result were relatively wide and
non-symmetrical around zero (CI = -3.757, 0.214), suggesting that a difference in C:N ratio
may exist but was not detected by our test. We found no differences among morphotypes in
their phenolic content (Fig 6d; t = -0.151, df = 8, p = 0.883; CI = -0.643, 0.564).
Effect of proximity to reef on consumption of macroalgae
Consumption of tethered Sargassum varied with increasing distance from the boundary
between the reef flat and lagoon, and the nature of the differences among distances varied
among sites (significant Site x Distance interaction; Fig 2.7, Table 2.5). Nearly 100% of the
tethered algae offered in the middle of the reef flat (-25 m) and at the reef flat/ lagoon
boundary (0 m) were consumed at all sites (all SNK comparisons between sites not
significant). We found variable differences in consumption among sites at distances between
20
Trophic effects through herbivory at Ningaloo Reef
2011
5 and 50 m from the reef flat/ lagoon boundary, but at all sites there was no consumption at
75 m (SNK comparisons between sites not significant). At two sites, there was either very
low or no consumption at 30 and 50 m from the reef flat/ lagoon boundary, but at the third
site there was still high levels of consumption at those distances (significant SNK
comparisons between site 1 and sites 2 and 3 at 30m and 50m).
21
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 2.1. Length of Sargassum myriocystum lateral branches consumed per hour (mean +
SE) at lagoon, reef flat and outer reef habitats at each of the experimental sites.
22
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 2.2. Variation between habitats in (a) total roving herbivorous fish biomass, (b) total
browsing fish biomass, (c) algal cover, (d) algal biomass, (e) coral cover, and (f) rugosity. Bars
represent means + SE.
23
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 2.3. Canonical analysis of principal coordinates (CAP) comparing community
assemblages of (a) all roving herbivorous fish, (b) all browsing fish, and (c) macroalgae
between sites (numbered icons) and habitats (symbols): Triangles facing upwards = Lagoon;
Triangles facing downwards = Outer reef; Squares = Reef flat habitat. Data were fourth-root
transformed prior to ordination.
24
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 2.4. (a) Logarithmic relationship between herbivory rates and algal cover. (b) Linear
relationship between herbivory rates and rugosity. All variables were averaged for each site.
25
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 2.5. (a) Biomass change and (b) nitrogen and (c) carbon/nitrogen ratio of Lobophora
variegata transplanted to reef flat and lagoon habitats in three experimental treatments
designed to manipulate access by herbivores (Caged, Open and Partially caged) after 6
weeks. Data pooled across the three sites, bars represent means + SE.
26
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 2.6. Results from reciprocal transplants of lagoon ‘ruffled’ and reef flat ‘decumbent’
morphotypes of Lobophora variegata: (a) rate of consumption, (b) nitrogen content, (c)
carbon: nitrogen ratio, and (d) phenolic content. Bars represent means + SE.
27
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 2.7. Length of Sargassum myriocystum lateral branches consumed after 48 hours
(mean + SE) at increasing distances from the reef flat/ lagoon boundary in the three
experimental sites.
28
Trophic effects through herbivory at Ningaloo Reef
2011
Table 2.1. PERMANOVA results on removal rates of Sargassum myriocystum lateral branches
between sites and habitats. Relevant significant probabilities are indicated in bold.
Source of variation
df
MS
Pseudo-
P
F
Site (S)
2
1.681
0.869
0.405
Habitat (H)
2
76.479
28.005
0.011
SxH
4
2.734
1.413
0.22
Residual
193
1.935
29
Trophic effects through herbivory at Ningaloo Reef
2011
30
31
Table 2.2. Results of the two factor PERMANOVAs assessing multivariate differences between sites and habitats in assemblages of (a) all roving herbivorous
fish, (b) browser herbivorous fish and (c) algae. All data were fourth-root transformed prior to analyses. Significant probabilities are indicated in bold.
2011
Trophic effects through herbivory at Ningaloo Reef
32
Table 2.3. Results of the two factor univariate analyses assessing differences in (a) all roving herbivores and (b) macroalgal browsers and (c) algal cover and
(d) total algal biomass between sites and habitats (a - b PERMANOVA; c – f ANOVA). Significant probabilities are indicated in bold. Pooling procedure was
used in accordance to Underwood (1997). Tested against the pooled term = S x H + Residual: * all roving herbivorous fish analysis MS = 1.93 x 108, df = 67;
† browser fish analysis MS = 4.34 x 106, df = 67; ‡ algae analysis MS = 1.99, df = 22
2011
Trophic effects through herbivory at Ningaloo Reef
33
Table 2.4. ANOVA results on (a) biomass change and PERMANOVA results on changes in (b) nitrogen and (c) C/N of Lobophora variegata transplanted
specimens in lagoon and reef flat habitats over the three sites. Pooling procedure was used in accordance to Underwood (1997). Relevant significant
probabilities are indicated in bold. * Tested against the pooled term = S x H x He + Plot (SxHxHe); MS = 48.5, df = 40
2011
Trophic effects through herbivory at Ningaloo Reef
Trophic effects through herbivory at Ningaloo Reef
2011
Table 2.5. ANOVA results on removal of Sargassum myriocystum lateral branches at three
sites and nine distances from the reef flat/ lagoon boundary. Relevant significant probabilities
are indicated in bold.
Source of variation
df
MS
F
P
Site (S)
2
1084.114
15.63
<0.001
Distance (D)
8
1561.181
6.91
<0.001
SxD
16
225.781
3.26
<0.001
Residual
54
69.364
34
Trophic effects through herbivory at Ningaloo Reef
2011
35
Trophic effects through herbivory at Ningaloo Reef
2011
APPENDIX Figure 1. Study area: Mandu and Maud sanctuary zones in Ningaloo Reef,
Western Australia. In Mandu, there were three sites within each habitat: lagoon (diamond
icons), reef flat (circle icons) and outer reef (square icons).
36
Trophic effects through herbivory at Ningaloo Reef
2011
Discussion
Herbivores operate in dynamic systems where they can both generate spatial heterogeneity
and respond to existing patterns in space. In this study, we found marked spatial variation in
the abundance, composition and consumption of macroalgae across a coral-reef seascape.
Spatial patterns in macroalgal consumption were best explained by differences in structural
complexity among habitats: herbivory was always highest in the most structurally complex
coral-dominated reef flat and outer reef habitats. In contrast, the cover and biomass of
macroalgae appeared to be themselves influenced by consumption patterns, with habitats
supporting high biomass of herbivores also supporting low algal abundance. Experimental
exclusion of herbivorous fish in different habitats supported the conclusion that these
consumers exert a strong influence on macroalgae in the structurally-complex reef flat
habitat, but not in the structurally-simple lagoon habitat. In addition, algal consumption
decreased to undetectable levels within 75 m of coral structure into the structurally simple
lagoon habitat, where highly palatable macroalgae were abundant. Although productivity and
nutritional quality of plants can both influence and be influenced by herbivory in terrestrial
systems [12,13,14], we found no evidence that these traits affect the distribution of herbivory
in the coral-reef seascape at Ningaloo Reef.
Seascape patterns in the distribution of macroalgae, herbivores and herbivory
The pattern of among-habitat differences in macroalgae cover and biomass and of biomass
and composition of roving herbivorous fish in Ningaloo Reef across distances of hundreds of
meters is remarkably similar to patterns observed across tens of kilometers in coral reefs
with different geomorphology, such as the Great Barrier Reef (GBR). Macroalgal cover in the
lagoon in Ningaloo ranged between 10-80%, values that are similar to inner shelf systems in
the GBR (36-66%), while the outer reef at Ningaloo (located about 1 km offshore) hosted
<10% algal cover, values that are more similar to mid-shelf or outer-shelf reefs located 50100 km offshore in the GBR (0-15%) (Wismer et al. 2009, Hoey & Bellwood 2010a). This
pattern is also apparent when comparing herbivorous fish biomass, which ranged from < 1
2
2
kg/ 125 m in the lagoon to up to 20 kg/ 125 m in the reef flat and outer reef, a difference
that is in the same order of magnitude as the disparity in roving herbivorous fish biomass
among inner-shelf and mid/outer-shelf reefs in the GBR (Wismer et al. 2009, Hoey &
Bellwood 2010a).
The distinct spatial patterns in consumption of algae described in this study are also similar
to the GBR (McCook 1997, Fox & Bellwood 2007), as well as to reefs found in the Caribbean
(Hay 1981b, Lewis & Wainwright 1985, Valentine et al. 2008), and in the Hawaiian Islands
(Stimson et al. 2001). Herbivory is always highest in coral-dominated habitats near or at the
reef crest, and decreases with either depth or distance towards the inner sections of
lagoons. This suggests that a similar process (or combination of processes) may be
controlling the distribution of herbivory in different coral-reef ecosystems, despite great
variations in their geomorphology and physical influences.
Relationship between patterns in herbivory and macroalgal distribution, productivity,
nutritional quality and palatability
We found a near-significant negative logarithmic relationship between algal cover and algal
consumption that suggests that the cover of macroalgae is reduced under high herbivory
conditions, but it also depends on other factors under low herbivory conditions. The
conclusion that this relationship is causal, rather than just correlative, is supported by the
growth in macroalgae with experimental exclusion of herbivores. We did not detect a similar
relationship between macroalgal biomass and herbivory, probably because much of the algal
biomass collected in our quadrats was present under coral plates and in small crevices that
37
Trophic effects through herbivory at Ningaloo Reef
2011
were effectively inaccessible to consumers, whereas algae surveyed as percentage cover
better reflect algae that is available to herbivores.
Spatial patterns of macroalgal consumption were not related to among-habitat differences in
macroalgal production (measured as biomass change) or nutrient content, since these algal
traits were similar in habitats with contrasting levels of herbivory. Experimental exclusion of
herbivores resulted in very similar growth of Lobophora variegata transplanted to the high
herbivory reef flat and low herbivory lagoon. Similarly, we did not find any among-habitat
differences in the nitrogen content of algae. We did, however, detect some differences
between habitats in the availability of nitrogen per unit carbon, which tended to be higher for
thalli placed in the reef flat (i.e. lower C:N ratio). Lower C:N ratios are generally associated
with higher palatability, and many marine herbivores are known to preferentially consume
and grow faster on diets with low C:N ratios (Barile et al. 2004, Van Alstyne et al. 2009).
However, in our feeding preference experiment between reef flat-decumbent and lagoonruffled L. variegata, the two morphotypes were equally consumed, despite near-significant
differences in C:N ratio. This contrasts with the findings of Coen and Tanner (Coen & Tanner
1989), who found striking differences in susceptibility to herbivory between the same
morphotypes in the Caribbean and suggested that their different morphologies were related
to differential grazing intensities in their respective habitats. However, these authors also
found greater chemical differences among morphotypes than detected in our study, which
may explain the discrepancy with our results.
Differences in the algal assemblages from habitats with contrasting levels of herbivory are
consistent with the inference that macroalgal consumers are influencing spatial patterns in
macroalgal community composition. The habitat with the lowest rates of herbivory (lagoon)
was characterised by high abundance of palatable Sargassum species, which are readily
eaten by macroalgal browsers worldwide when accessible (pers. obs.; (Randall 1967,
McClanahan et al. 1994, Hoey & Bellwood 2009, Hoey 2010) and are actively selected by
siganids (Mantyka & Bellwood 2007a). Of the algae that characterised reef flat habitats,
Turbinaria ornata is considered unpalatable (Bittick et al. 2010), while Lobophora variegata is
consumed by browsers to varying degrees (pers. obs.; (Randall 1967, Lewis 1985, Steinberg
& Paul 1990, Diaz-Pulido et al. 2009, Bennett et al. 2010). Outer reef habitats with similar
levels of herbivory were characterised by the red algae Amphiroa sp. and Amansia sp., both
of which are actively avoided by some siganids (Von Westernhagen 1974, Mantyka &
Bellwood 2007a). These findings are consistent with other studies that show a restriction of
palatable species to areas of the reef with low levels of herbivory (Hay 1984, McCook 1997).
Feedback mechanisms between herbivory and algae
In many terrestrial systems, herbivores have a positive effect on the plants they consume
through enhancing nutrient recycling and availability, a feedback mechanism known as
grazing optimisation (McNaughton et al. 1997, De Mazancourt et al. 1998, Augustine et al.
2003). However, in our cage experiments macroalgae that were exposed to herbivores had
the lowest nitrogen concentrations. Our results are consistent with several marine studies
that show a short-term reduction in nitrogen content in seagrasses grazed by fish and
urchins (Alcoverro & Mariani 2005) (Vergés et al. 2008). This lack of a fertilisation effect may
be partly due to the fact that the localised input of nutrients that occurs in terrestrial systems
via the faeces and urine of herbivores is likely to be reduced in the marine environment,
generally due to the dilution and dispersion of nutrients via water movement. Indeed, the
only marine examples where optimisation effects have been recorded through excretion of
nitrogenous wastes of herbivores are from shallow, poorly flushed systems (Fong &
Desmond 1997). Other examples of grazing optimization effects in the marine environment
come from systems dominated by specialist herbivores such as turtles, who generally raise
the nitrogen content of seagrasses through increasing the proportion of nutrient-rich new
foliage by repeated cropping (Bjorndal 1980, Zieman et al. 1984, Aragones et al. 2006).
Additionally, herbivores can also indirectly enhance the nitrogen content of macrophytes by
38
Trophic effects through herbivory at Ningaloo Reef
2011
inducing bacterial nitrogen fixation either through sediment disturbance (e.g. effects of
dugongs on seagrass meadows; (Aragones et al. 2006) or by removing algal recruits and
facilitating dominance by rapidly colonising nitrogen-fixing cyanobacteria (Munday et al. ,
Wilkinson & Sammarco 1983). While the lack of nitrogen enhancement of transplanted algae
in our cage experiment could be partly due to the short duration of the trial (6 weeks), the
fact that we found no differences in the nitrogen content of Lobophora variegata specimens
from habitats with contrasting levels of herbivory (lagoon and reef flat morphotypes)
indicates that potential differences in nitrogen fixation or other nitrogen uptake mechanisms
between habitats are not having an effect on macroalgal nitrogen content in Ningaloo Reef.
Herbivory patterns explained by structural complexity
In this study, structural complexity was identified as the key feature mediating spatial
patterns of macroalgal consumption by fish. This conclusion is supported by three lines of
evidence: (1) different outcomes from experimental exclusion of herbivorous fish in
structurally-complex reef habitat and structurally-simple lagoon habitat, (2) a strong linear
relationship between macroalgal consumption and structural complexity; and (3) a decrease
in herbivory with increasing distance from structurally-complex reef habitat. These results are
consistent with other studies that have shown an increase in herbivore density and grazing
rates with topographical complexity (Hay et al. 1983b, McCook 1997, McClanahan 1999,
Valentine et al. 2008). Although there is a lack of experimental studies identifying the specific
causes that link herbivory and structure, availability of shelter or refuges, increased diversity
of microhabitats and resource partitioning are thought to be key influences (Vanderklift et al.
2007). Complex habitats can reduce predation by providing shelter (Hixon & Beets 1993),
lower competition through increased niche availability (Jones & Syms 1998, Munday &
Jones 1998), and provide specific settlement habitat for larvae (Jones et al. 2004).
Nevertheless, greater structural complexity is not associated with higher herbivory at all
spatial scales. For example, herbivory is lower within branching coral habitats that are highly
structured at small scales (cm) than in nearby planar coral habitats (Bennett et al. 2010).
Understanding the mechanisms that drive spatial patterns of ecological processes in coral
reefs is particularly important for the management of these systems, because the ability of
individual coral reefs to reorganise and maintain ecosystem function following disturbance is
considered to strongly depend on the matrix of adjacent reefs and habitats in the
surrounding seascape (Peterson et al. 1998, Nystrom & Folke 2001). This study highlights
the importance of structural complexity in establishing spatial patterns of macroalgal fish
herbivory, an ecological process of key importance that can reverse phase shifts when algae
overgrow corals following disturbances (Bellwood et al. 2006a). Structural complexity thus
emerges as a critical feature of reefs that is essential for the healthy functioning of the
ecosystem.
39
Trophic effects through herbivory at Ningaloo Reef
2011
CHAPTER 3. Herbivore diversity on coral reefs: a
transcontinental comparison
Adriana Vergés, Scott Bennett, David Bellwood
Introduction
Herbivory is a key ecological process in coral reefs that supports intricate food webs and
strongly contributes to the resilience of these systems, i.e. their ability to reorganise and
maintain ecosystem function following disturbance (Bellwood et al. 2004; Mumby et al.
2006). In recent decades roving herbivorous fishes have been identified as key elements of
coral reef communities, and overfishing of these consumers is considered to be a significant
factor contributing to reef degradation worldwide. This is often linked to phase shifts from
coral to macroalgal dominance (Hughes 1994; McClanahan et al. 2001; Graham et al. 2006).
However, roving herbivorous fishes do not constitute an ecologically uniform group, but
rather comprise an agglomerate of species with widely varying feeding modes and diets
(Choat et al. 2002; Choat et al. 2004; Crossman et al. 2005) that have been broadly
categorised into grazer and browser functional groups (Bellwood et al. 2004; Green and
Bellwood 2009; Hoey and Bellwood 2010a). The grazer functional group, which includes
excavating and scraping species (primarily parrotfishes and acanthurids), is largely restricted
to consuming algal turfs and the associated material in the epilithic algal matrix (EAM, sensu
Wilson et al. 2003), and can therefore only limit macroalgal abundance by consuming
recruits (Bellwood et al. 2004, Green and Bellwood 2009). In contrast, browsers are able to
remove large erect macroalgae and thus have the potential to reverse phase shifts once
macroalgae are established on reefs (Bellwood et al. 2006a).
An extensive body of literature from a wide range of coral reef systems shows that
macroalgal browsers are highly selective, and that most species feed on a small subset of
the available algal species (Randall 1967; Wylie and Paul 1988; Paul et al. 1990; Burkepile
and Hay 2008). Feeding selectivity has been linked to chemical and physical defences
developed by many tropical algal species as a defence against herbivory (Wylie and Paul
1988; Paul et al. 1990; Targett and Targett 1990). In contrast, other tropical algal species
that are highly susceptible to herbivory largely depend on spatial refuges to persist and are
therefore only abundant in habitats characterised by low herbivore biomass or accessibility
(Hay 1981b, 1984, Lewis 1986, Duffy and Hay 1990).
On the Great Barrier Reef (GBR), transplant experiments have shown that the abundance
and distribution of Sargassum species is strongly influenced by herbivory (McCook 1997).
Furthermore, herbivore-exclusion experiments have shown that this genus dominates
macroalgal biomass in the absence of larger herbivorous fish, and has catastrophic
community-level effects because it depresses the fecundity, recruitment and survival of
corals (Hughes et al 2007). Despite Sargassum being considered susceptible to herbivory on
the GBR, recent studies in this region have shown that removal of this macroalga is often
dominated by only one or two browsing species (Bellwood et al. 2006b; Fox and Bellwood
2008; Cvitanovic and Bellwood 2009; Hoey and Bellwood 2009). This limited redundancy
among macroalgal consumers highlights the potential vulnerability of coral reefs to
disturbance and stresses the need to assess the functional role of individual species of
herbivores (Hoey and Bellwood 2009). However, our knowledge of such species-specific
patterns in macroalgal consumption is currently limited geographically, and there is a need to
determine whether the patterns observed in specific reefs are applicable at a broader scale.
In this study, we directly compare species-specific rates of Sargassum consumption in the
southern GBR (east coast of Australia) with a coral reef system located at similar latitude in
the west coast of Australia: the Ningaloo Reef. Video cameras were used to quantify rates of
40
Trophic effects through herbivory at Ningaloo Reef
2011
macroalgal consumption by individual species and underwater censuses were performed to
compare herbivorous fish communities in the two systems.
Materials and Methods
Study locations
This study was conducted between December 2008 and February 2009 in the Keppel
Islands Group (23º 109S, 151º 009E) on the GBR (East Australia) and on Ningaloo Reef
(22º 07S, 113º 52E) in Western Australia (Figure 3.1). The Keppel Islands Group includes 15
islands located about 18 km from mainland Australia in the southern inshore GBR, and are
influenced by the Fitzroy river catchment. The Ningaloo Reef is a fringing arid-zone reef
approximately 290 km in length that forms a discontinuous barrier adjacent to the North West
Cape. In Ningaloo, expansive coral growth occurs within 100s of meters from the mainland
and the reef crest is constantly flushed by high wave energy. At each study region, the study
took place in habitats characterised by the highest herbivory levels, based on published
evidence, and the most similar benthic structure, based on preliminary cross-habitat surveys.
At the Keppel Islands, the study took place in the reef crest zone (~ 3 m depth at high tide).
This zone displays the highest rates of herbivory and herbivore biomass across the fringing
reef profile (Fox and Bellwood 2007) and is locally characterized by high coral cover (55.6 +
4.0 %; mean + SE). Ningaloo Reef does not have accessible reef crests due to wave energy,
and the study was instead performed in the reef flat back reef habitat (~2 m depth at high
tide, herafter referred to as ‘reef flat’), which is also characterised by high herbivore biomass
and the highest rates of herbivory across the fringing reef profile (Johansson et al. 2010;
Vergés et al. Chapter 2) and supports high coral cover (40.7 + 3.7 %; mean + SE).
Within each region, we selected three representative reefs (hereafter referred to as
locations) that were all situated within sanctuary zones, to minimise the potential effect of
extractive activities. The three Keppel Island locations were Olive Point (23° 09S 150° 55E),
Middle Island (23° 10S, 150° 55E) and Halfway Island (23° 11S 150° 58E). The three
Ningaloo locations were Mangrove Bay (21°58S, 113° 54E), Mandu (22°05S, 113° 52E), and
Osprey (22°14S, 113° 52E). Within each location, two sites were haphazardly selected about
100 m apart.
Macroalgal assays and video analysis
To compare the fish community responsible for macroalgal removal in the different regions,
locations and sites, the brown alga Sargassum myriocystum was used as a bioassay
because it is a dominant species in both regions, is readily identifiable in the field, and pilot
studies indicated that it was palatable and readily eaten by fish within a few hours.
At each site, ten S. myriocystum assays (ca. 230 g) were haphazardly deployed on the reef.
Five individual assays were tethered to the dead coral substratum using a rubber band and
gardening wire, and five of the assays were protected from herbivores in cages (50 x 50 x 50
2
cm; 1.44 cm mesh size) to control for any biomass changes not due to herbivory by fish
(e.g. handling losses and algal detachment due to water movement). Algae were deployed
for approximately 4.5 hours between 8am and 4pm over three consecutive days. Fresh
weight (to the nearest 0.1 g) was recorded before and after deployment. Algal biomass
losses due to herbivory were calculated by randomly pairing individual treatment and control
specimens (Roa 1992) and substracting the change in biomass of the treatment specimen
from the change in biomass of the control specimen (uncaged – caged). Average biomass
41
Trophic effects through herbivory at Ningaloo Reef
2011
changes in control specimens were 10.15% of initial weight in the Keppels region and
18.66% of initial weight in Ningaloo (n = 90).
Video analysis
Two of the five treatment assays deployed at each site were filmed using a stationary
underwater video camera (either a Sony DCR-HC1000E or a Sony HDR SR12 in an
underwater housing) following the techniques of Hoey and Bellwood (2009). The three days
at each site yielded approximately 81 hours of footage per region. Species-specific rates of
macroalgal consumption were quantified following the methods detailed by Bellwood et al.
(2006a). The total number of bites per fish species and size (total length, TL) was recorded
from the video footage for each sampling period. To account for variation in bite size related
to differences in body size, the midpoint of each size class was used to calculate massstandardised estimates of bite ‘impact’ for each fish species (total number of bites × body
mass in kilograms) based on established length weight relationships from the literature
(following Bellwood et al. 2006b). Forays, where rapid consecutive bites by an individual fish
took place without a discernable pause, were conservatively classed as a single bite
(Bellwood and Choat 1990).
Multivariate differences in the assemblages of fishes feeding on the Sargassum myriocystum
bioassays were calculated using a three-way permutational analysis of variance
(PERMANOVA) with the following factors: Region (2 levels), Location (3 levels, random,
nested within Region), and Site (2 levels, random, nested within Location and Region). The
Bray-Curtis distance was our metric in the multivariate analyses. Non-metric
multidimensional scaling (nMDS) was used to produce two-dimensional ordinations of the
similarities between multivariate fish samples. The similarity percentages procedure
(SIMPER, Clarke 1993) was used to determine the fish taxa that contributed most strongly to
dissimilarities between multivariate samples from the different regions. The contribution of
each taxon was evaluated using the ratio of the mean overall dissimilarity between sets of
samples and the standard deviation of this contribution (mδi/SD[δi]). Taxa were considered
‘important’ if this ratio was higher than 1 (i.e. the mean contribution was higher than the
variation). All multivariate statistical analyses were performed using Primer-E v6 software
(Clarke and Gorley 2006) with the PERMANOVA+ add-on package (version 1.0.1;(Anderson
et al. 2008a)).
Relationship between bite rates and macroalgal removal
In order to identify the fish species that contributed most strongly to macroalgal removal on
Ningaloo Reef and the Keppel Islands, we first selected the herbivorous fish species that
were responsible for > 5 % of bites in each region (four species per region, see Results). We
then used simultaneous multiple regression to describe the relationship between algae
removed in the filmed bioassays (dependent variable) and the corresponding massstandarised feeding rates for that particular filmed replicate of the four herbivorous fish
species and all other species pooled together (predictor variables; n = 36 filmed replicates
per region; one analysis per region). Multiple regression analyses were performed using R
software (Version 2.9.0, R Development Team 2009).
Distribution of herbivorous fishes
The roving herbivorous fish community from each region, location and site was censused
using standard underwater visual surveys. Fishes were counted on six replicate 10 minute
timed swims per site during daylight hours by divers on SCUBA (avoiding 2 hours before and
after sunrise) (Bellwood and Wainwright 2001). Fish counts were performed swimming at a
constant speed and counting and estimating the size of fish within a 4 m wide transect (all
42
Trophic effects through herbivory at Ningaloo Reef
2011
censuses performed by SB). The length of each transect was subsequently measured using
tapes (116 ± 8.7 m mean ± SE). Fishes were identified to species level and their total length
was estimated in 5 cm size categories. Density estimates were converted to biomass using
b
the allometric length-weight conversion W= a * TL , where W is weight in grams, TL is total
length and parameters a and b are constants obtained from the literature (Froese and Pauly
2005). Counts were restricted to fishes over 10 cm TL from the families Acanthuridae,
Siganidae, Kyphosidae and Labridae (parrotfishes). Individuals belonging to the species
Acanthurus auranticavus, A. grammoptilus and A. blochii were grouped as Acanthurus spp.
due to difficulties in identification.
Multivariate differences between the fish assemblages counted in the underwater
censes were calculated using a three-way PERMANOVA as described above. nMDS plots
produced two-dimensional ordinations of the similarities between multivariate fish samples,
and SIMPER was used to determine the fish taxa that contributed most strongly to
dissimilarities. Univariate differences in total herbivorous fish biomass were calculated with
the same three-way factorial design using the statistical package GMAV (coded by A. J.
Underwood and M. G. Chapman, University of Sydney, Australia).
Results
Video analysis
We found strong regional differences in the diversity of fish species observed feeding on the
algal bioassays, which was much higher in Ningaloo Reef (23 species) than in the Keppel
Islands (8 species) (Figure 3.2). The number of bites recorded in Ningaloo Reef (15,792
bites) was also much higher than that recorded in the Keppel Islands over the same time
period (1,085 bites). In the Keppel Islands, four species were responsible for over 95% of all
mass standarised bite rates: Kyphosus vaigiensis (68.2%), Naso unicornis (10.9%), Siganus
doliatus (10.5%) and Siganus canaliculatus (8.8%). The other Keppel Island fishes (four
species) individually accounted for <5% of mass standarised bites. In Ningaloo Reef, the
following four species accounted for over 85% of all mass standarised bite rates: Scarus
schlegeli (29.5%), Kyphosus vaigiensis (24.3%), Naso unicornis (18.0%) and Scarus
ghobban (10.3%). A further 19 species accounted for the rest of the bites, with each species
being responsible for < 5% of mass standarised bites individually.
We also found strong differences in the mass standarised bite rates taken by the fish
assemblages observed feeding on the macroalgal assays (Table 3.1a), which were clearly
plotted on the nMDS ordination as two separate groups (Figure 3.4). We detected
differences between the fish assemblages that fed in the different sites within each location,
but not between locations within the two regions (significant Site (Location (Region)) effect in
Table 3.1a). The SIMPER procedure identified two fish species that were characteristic of
the Ningaloo assemblage of macroalgae-feeding fishes: Scarus schlegeli (mean similarity/
standard deviation mδi/SD[δi] = 1.55) and Scarus ghobban (mδi/SD[δi] = 1.11). No species
were identified as characteristic of the Keppels Islands region. Similar statistical results were
obtained whether we analysed mass standarised bite rates (total number of bites × body
mass in kilograms per 4.5 h) or bite rate data (total number of bites per species per 4.5 h;
statistical results not shown).
Relationship between bite rates and macroalgal removal
In the Keppel Islands, the mass standarised bites of Kyphosus vaigiensis, Naso unicornis,
Siganus doliatus, S. canaliculatus (the four species individually responsible for > 5% bites)
and all other species pooled, explained about 56% of the variation in the loss of algal
43
Trophic effects through herbivory at Ningaloo Reef
2011
2
biomass from our assays (F 5, 30 = 7.6, p < 0.001, adjusted R = 0.485; Table 3.2. However,
partial regressions indicated that only the mass standarised bite rates of Naso unicornis at
Keppel Islands had a significant effect on algal biomass loss of the filmed assays (Table
3.2). The relationship between macroalgal biomass loss in Ningaloo Reef and the mass
standarised bite rates of Scarus schlegeli, K. vaigiensis, N. unicornis, S. ghobban and all
2
other species pooled was marginally not-significant (F 5, 30 = 2.18, p = 0.0833, adjusted R =
0.144).
Distribution of herbivorous fishes
We found strong differences in the herbivorous fish community composition between the two
regions and between locations within each region (Table 3.1b). Regional differences were
clearly displayed as two separate groups in the nMDS plot (Figure 3.5). SIMPER analyses
identified Siganus doliatus as the only species characteristic of the Keppel Islands
(mδi/SD[δi] = 1.23). Five species characterised Ningaloo Reef samples: Chlorurus sordidus
(mδi/SD[δi] = 3.64), Acanthurus triostegus (mδi/SD[δi] = 2.68), Scarus schlegeli (mδi/SD[δi] =
2.19), initial phase parrotfish (scarid IP; mδi/SD[δi] = 1.58), and S. ghobban (mδi/SD[δi] =
1.23).
There were striking differences in species diversity, with 33 species being censed in
Ningaloo Reef compared with only 16 in the Keppel Islands (Figure 3.6). Similarly, we
detected significant regional differences in total biomass of all roving herbivorous fish, with
Ningaloo Reef biomass values being over 13 times those of the Keppel Islands (Figure 3.3;
Table 3.1c). In Ningaloo Reef, we found differences in total fish biomass between locations
(SNK post-hoc tests), but not between sites in any of the two regions (Table 3.1c).
44
6
Site
(Reg))
2
2
4
Location (Reg)
Residual
1
Region
(Loc
d
f
Source
959.5
8
2149.
6
3392.
7
3407
6
MS
2.2401
1.5847
10.156
Pseudo
-F
(a) Feeding fish community
0.01
0.15
2
0.00
2
P
5
9
6
4
1
d
f
45
1051.
4
1467.
1
5625
6664
3
MS
1.3954
3.8345
11.855
Pseudo
-F
0.05
7
0.00
1
0.00
1
P
(b) Fish community composition
6
0
6
4
1
d
f
1.437
0
1.286
6.334
85.95
8
MS
0.89
4.93
13.57
F
0.50
5
0.04
2
0.02
1
P
(c) Total herbivorous fish biomass
0.50
5
0.04
2
0.02
1
P
Table 3. 1. Results of the three factor analyses of variance assessing differences between regions, locations and sites in (a) Mass standarised bite rates
from the fish community feeding on the algal bioassays (PERMANOVA), (b) Roving herbivorous fish community composition (PERMANOVA) and (c) Total
roving herbivorous fish biomass (ANOVA). Significant probabilities are indicated in bold.
2011
Trophic effects through herbivory at Ningaloo Reef
Trophic effects through herbivory at Ningaloo Reef
2011
Table 3. 2. Results of multiple regression analysis on the relationship between algae biomass
loss in the Keppel Islands and the standarised bite rate of the four species responsible for >5%
2
of all bites and all other species pooled together. Overall model: Adjusted R = 0.48, F5, 30 =
7.6, p < 0.001. Significant probabilities are indicated in bold.
Source
Estimate
Estimate SE
t
p
N. unicornis
95.225
36.172
2.633
0.013
K. vaigiensis
23.646
15.997
1.478
0.150
S. doliatus
8.251
20.090
0.411
0.684
S. canaliculatus
-6.100
19.627
-0.311
0.758
Sum all other species
-15.785
46.709
-0.338
0.738
46
47
Figure 3.1. Map of the two study regions, the Keppel Islands in the southern Great Barrier Reef and Ningaloo Reef, showing the location of the study sites.
2011
Trophic effects through herbivory at Ningaloo Reef
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 3.2. Total number of mass standarised bites (log transformed) taken by of the
herbivorous fish assemblages feeding in each region over 4.5 hours (n = 6).
48
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 3.3. Variation between regions and locations in total roving herbivorous fish biomass (n
= 6). Sites were not significantly different from each other and have been pooled.
49
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 3.4. Non metric multidimensional scaling plot (nMDS) comparing the herbivorous fish
assemblages feeding on the algal bioassays between regions (symbols) and locations (n = 6).
Data were fourth-root transformed prior to ordination.
50
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 3.5. Non metric multidimensional scaling plot (nMDS) comparing the herbivorous fish
assemblages between regions (symbols) and locations (n = 6). Data were fourth-root
transformed prior to ordination.
51
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 3.6. Herbivorous fish species total abundance at each region measured through
underwater visual censuses (n = 6).
52
Trophic effects through herbivory at Ningaloo Reef
2011
Discussion
We found strong differences between Ningaloo Reef and the Keppel Islands in both the
diversity of species observed feeding on Sargassum and in the species composition of the
roving herbivorous fish communities. The diversity of fish species recorded biting the algal
bioassays in Ningaloo Reef (23 species) was much higher than in the Keppel Islands (8
species) and is one of the highest recorded in coral reefs to date (cf. 20 species in Bennett
and Bellwood (2011) on the northern GBR). Video observations from the Keppel Islands
confirm the role of a small number of key species previously identified as important
macroalgal feeders in GBR studies, whereas consumption of macroalgae in Ningaloo Reef
was spread over a larger number of less-dominant species.
In previous studies that have aimed to identify the key fish species responsible for
consumption of Sargassum in the Great Barrier Reef, four different species have been
identified as important (Platax pinnatus, Kyphosus vaigiensis, Siganus canaliculatus, and
Naso unicornis), and a common pattern has emerged whereby a single species has
dominated feeding at the local level (Bellwood et al. 2006b; Mantyka and Bellwood 2007b;
Fox and Bellwood 2008; Cvitanovic and Bellwood 2009; Hoey and Bellwood 2009; Lefèvre
and Bellwood 2010). All these previous studies were performed in the central to northern
regions of the GBR. Our Keppel Islands results provide further confirmation of this pattern by
highlighting the key role of one species, N. unicornis, in removing Sargassum in these
southern GBR inshore reefs. Our results from Ningaloo Reef, however, provide a contrasting
pattern.
In this western reef, we found a large number of fish species feeding on the algae bioassays,
but none of them dominated the macroalgal removal process. K. vaigiensis and N. unicornis
were two of the species responsible for the highest mass standarised bite rates in both
Ningaloo (24.3% and 18.0%) and the Keppel Islands (68.2% and 10.9%, respectively) and
have been recognised as important macroalgal consumers in recent studies (Cvitanovic and
Bellwood 2009; Hoey and Bellwood 2009; Lefèvre and Bellwood 2010). However, the other
two species responsible for the highest mass standarised bite rates in Ningaloo Reef
(Scarus schlegeli and Scarus ghobban, 29.5% and 10.3% of all bites, respectively) have not
been previously identified as important macroalgal feeders in any other reef system.
Nevertheless, despite the very large number of bites taken by these two parrotfish species,
we found no evidence that their biting had a measurable net effect on algal biomass loss in
the regression analyses. It is quite likely that these two parrotfish species were not feeding
on the macroalgae thallus itself, but were instead feeding on epibiota and/or on surface
detritus as previously stated in Scarus rivulatus on the GBR (Lefèvre and Bellwood 2010).
Indeed, S. rivulatus, like S. schlegeli and S. ghobban, have all been identified as scrapers
(i.e. consumers of EAM) in studies based on their jaw morphology and field observations
(Bellwood and Choat 1990).
We found striking differences in the species composition, species richness and total biomass
of roving herbivorous fish between the two regions. Ningaloo Reef hosted a diverse
assemblage of roving herbivores, with biomass values that were an order of magnitude
higher than in the comparatively depauperate fish assemblages of the Keppel Islands.
Differences in herbivorous fish communities of a similar magnitude are also observed across
different continental regions of the GBR, with inshore reefs having significantly lower
abundance and diversity of roving herbivores than mid-shelf and outer-shelf reefs (Williams
and Hatcher 1983; Russ 1984; Wismer et al. 2009). Thus, despite Ningaloo Reef being
found in close proximity to the mainland (within meters), its roving herbivorous fish
community is more comparable to mid-shelf and outer-shelf reefs in the GBR than to inshore
reefs such as the Keppel Islands (Johansson et al. 2010). This is probably due in great
measure to a host of physical conditions that strongly limit the influence of the mainland on
Ningaloo Reef. This western coast reef is constantly flushed by high wave energy and is
located in an arid zone where evaporation rates far exceed annual rainfall, hence minimising
terrestrial run-off and its effect on turbidity and sediment load (Cassata and Collins 2008).
Additionally, anthropogenic impacts are extremely low in Ningaloo Reef, with low human
populations, no agricultural activities, and very limited fishing activity. In contrast, inshore
53
Trophic effects through herbivory at Ningaloo Reef
2011
GBR reefs are strongly influenced by increasing sediment, nutrient and pesticide loads from
several degraded river catchments due to agricultural activities and other land-use practices
(McCulloch et al. 2003; Devlin and Brodie 2005).
Overall, our results confirm the key role of some of the herbivorous fish species identified as
important in previous studies (Naso unicornis and Kyphosus vaigiensis), but suggest that,
although these species are important on the GBR, the Keppels may have limited resilience
when compared to other reefs such as Ningaloo, where functional redundancy among
macroalgal consumers appears to be broader. This is consistent with recent experimental
evidence that shows that higher diversity of herbivorous fish can significantly lower
macroalgal abundance in coral reefs (Burkepile and Hay 2008, 2010), and with a new study
that integrates a long-term data set of field surveys in the GBR and further confirms a strong
association between low fish herbivore diversity and a coral-macroalgal phase-shift (Cheal et
al. 2010).
54
Trophic effects through herbivory at Ningaloo Reef
2011
CHAPTER 4. Rates of Sargassum consumption and identity of
herbivores vary across hundreds of kilometres along a
continental fringing coral reef
Peter Michael, Adriana Vergés, Glenn Hyndes and Mathew Vanderklift
Introduction
Herbivory is an important ecological process that can control rates of primary productivity
and the abundance of autotrophs in a wide range of terrestrial (Frank et al. 2002) and
aquatic (Burkepile and Hay 2006) ecosystems. In coral-reef ecosystems, high rates of
herbivory by fishes is often considered essential for the persistence of a coral-dominated
state (Hughes et al. 2003, Bellwood et al. 2004), with these herbivores playing a critical role
in mediating the competitive interactions between corals and benthic algae (Mumby 2009).
Indeed, the removal of herbivores from an area of reef disturbance, be it through overfishing
(Jackson et al. 2001), disease (Hughes 1994) or experimental exclusion (Hughes et al.
2007), often results in transitions from a coral-dominated state to one dominated by erect
stands of macroalgae (Done 1992, Ledlie et al. 2007).
Herbivory by fishes within coral-reef ecosystems does not, however, represent a uniform
process. Different ‘nominally’ herbivorous coral-reef fishes (sensu Choat et al. 2002) can
exhibit marked variation in feeding morphology (Bellwood and Choat 1990), behaviour (Fox
et al. 2009), dietary preferences (Choat 1991) and subsequent impacts on the underlying
substrata (e.g. Bonaldo and Bellwood 2009). This variation can result in a suite of species
playing different, but often complementary, roles in maintaining the structure and function of
coral reefs (Burkepile and Hay 2008). Functionally, herbivorous fishes may be broadly
classified into two distinct groups: ‘grazers’ and ‘browsers’. Grazing fishes, including
scrapers, excavators (Bellwood and Choat 1990, Bonaldo and Bellwood 2010) and other
detritivores (e.g. Fox et al. 2009), which target various components of the epilithic algal
matrix (EAM; sensu Wilson et al. 2003) and therefore influence macroalgal abundance only
through the removal of algal recruits (Bellwood et al. 2004, Bonaldo and Bellwood 2010). In
contrast, macroalgal browsers are responsible for the removal of substantial portions of erect
macroalgae from reefs (Bellwood et al. 2006b, Hoey and Bellwood 2009). The role of this
browsing functional group of herbivorous fishes therefore represents a separate and crucial
ecological process, particularly in the context of reef degradation and the ability of a reef
system to reverse a ‘phase shift’ once erect canopy-forming macroalgae have been
established following a disturbance (Bellwood et al. 2006b, Hughes et al. 2007). Thus,
understanding variation in the role macroalgal consumers can play within a reef system is
fundamental in understanding the processes that structure coral-reef macroalgal
communities, and ultimately, the health and resilience of a reef.
Traditionally, studies quantifying macroalgal herbivory by fishes have either: ignored the
identity of the consumer (Lewis and Wainwright 1985, Hay 1981a); inferred a species’
function based on its presence in an area (Williams and Polunin 2001); used feeding
experiments in aquaria (Targett and Targett 1990); or undertaken gut-content analyses
(Clements and Choat 1997). The recent use of remote video cameras, however, has
provided us with a new and useful insight into the varying algal-removal roles of herbivorous
fishes in coral reefs (Mantyka and Bellwood 2007a, Cvitanovic and Bellwood 2009, Lefevre
and Bellwood 2011), often revealing patterns different to those based on inference alone
(e.g. Bellwood et al. 2006b, Fox and Bellwood 2008, Hoey and Bellwood 2009). Within the
Indo-Pacific region, much of this direct understanding of variation in rates of macroalgal
removal is confined to discrete points within expansive coral reef systems. For example,
55
Trophic effects through herbivory at Ningaloo Reef
2011
within the Great Barrier Reef (GBR), herbivory studies are largely restricted to local reefs
around two islands within a system that stretches ca. 2600 km (Bellwood et al. 2006b,
Mantyka and Bellwood 2007b, Fox and Bellwood 2008, Cvitanovic and Bellwood 2009; Hoey
and Bellwood 2009, 2010a; but see Bennett and Bellwood 2011). While many of these
spatially restricted studies share a common pattern whereby the process is dominated by a
select few species within a relatively diverse fish assemblage (e.g. Hoey and Bellwood
2009), our understanding of how well these patterns can be generalised on reefs elsewhere
is still in its infancy. In the present study, we therefore quantify rates of macroalgal herbivory
by fishes at scales relevant to the spatial extent of Ningaloo Reef, a fringing coral-reef
system abutting a continental coast. We use Sargassum myriocystum (hereafter referred to
as Sargassum) tethers, the dominant alga in Ningaloo Reef in terms of biomass (Vanderklift
and Vergés unpubl. data), and remote video cameras to quantify both local- (within-reef, <1
km) and broad-scale (regional, 50-300 km) variability in rates of herbivory and the identity of
herbivores. In doing so, we also examine the relationship between Sargassum consumption,
the composition of roving herbivorous fish assemblages and the feeding behaviour of fishes
at these representative scales. Situated along the sparsely populated north-west cape of
Western Australia, the Ningaloo Reef is a relatively intact arid-zone nearshore coral-reef
system that provides a unique opportunity to investigate representative spatial variability in
algal-herbivore interactions without the potentially confounding influences of poor water
quality or overfishing.
Materials and methods
Study locations
This study was conducted during April and May 2009 on Ningaloo Reef, a fringing coral-reef
that forms a discontinuous barrier approximately 290 km in length along the north-west cape
of Western Australia (Figure 4.1). Within the reef, we selected five sanctuary (no-take) zones
encompassing most of the latitudinal extent of the Ningaloo Marine Park (Bundegi 21°51’S,
114°10’E; Mandu 22°05’S, 113°52’E; Point Cloates 22°44’S, 113°39’E; Maud 23°05’S,
113°44’E; Gnaraloo Bay 23°45’S, 113°31’E). Within each region, three reef-flat sites were
randomly selected at 2-3 m depths and spaced approximately 300 m apart (Figure 4.1). All
sites were located in coral-dominated reef-flat habitat, since this habitat displays amongst
the highest levels of herbivory and herbivorous fish density within the Ningaloo Reef
(Johansson et al. 2010, Chapter 2).
Macroalgal tethers
Sargassum myriocystum was used to quantify variability in macroalgal removal by fishes
since it represents a conspicuous, palatable (Vergés unpubl. data) and readily identifiable
macroalga present in the adjacent lagoon habitats of most study regions. Importantly,
Sargassum is often the dominant taxa following experimental exclusion of herbivores in
many Indo-pacific reefs such as the GBR (Hughes et al. 2007) and Ningaloo Reef (Webster
2007). Sargassum thalli were collected from lagoon habitats adjacent to the reef-flat in each
region, with the exception of Gnaraloo Bay where there were no large algal beds and algal
material was instead collected from the closest region (Maud) where Sargassum occurred.
Care was taken to minimise damage during algal collection by maintaining holdfasts intact.
Thalli were transported back to the laboratory in catch bags and intermittently submerged in
seawater in order to retain their moisture and turgidity. Prior to processing in the laboratory,
the basal portions of the plants were bundled with a rubber band and excess water was
removed using 10 standard spins (approximately 30 s duration) on a pull-cord salad spinner.
The fresh weight of the tethers was then recorded using digital scales before being assigned
a random identification label. Initial weight of tethers ranged from 300 to 360 g, with a mean
mass of 332 g ± 2 g (± standard error) over the study period (n = 360 algal assays).
56
Trophic effects through herbivory at Ningaloo Reef
2011
Processed material was maintained in catch bags in the ocean overnight until used for
tethering experiments, with all plants being used within 18 h of initial collection.
Within each reef-flat site, six Sargassum replicates were haphazardly tethered at least 5 m
apart to bare substratum using plastic-coated garden wire. Tethering wire and identification
labels were concealed as much as possible to avoid any potential feeding deterrence effects
to herbivorous fishes. No Sargassum tethers were placed within or near damselfish
(Pomacentridae) territories, since territorial damselfishes can reduce the feeding rates and
foraging effectiveness of other herbivorous fishes (Foster 1985). A further six control
replicates were deployed and protected from all herbivores by individual exclusion cages (50
-2
cm L × 50 cm W × 75 cm H). A mesh size of 1.5 cm was used to minimise caging artefacts
such as a reduction in water flow, while still restricting access by fishes. Each caged control
tether was deployed near a treatment tether, forming a total of six paired treatment-control
replicates for each sampling period. These six paired treatment-control Sargassum tethers
were used to determine changes of biomass experienced by the algae over the deployment
period due to factors other than herbivory (e.g. handling loss and abrasion). The physical
proximity of the treatment-control pairs ensured that the variance in mass loss attributed to
external factors jointly affected both the control and the treatment algae within a replicate
pair (Prince et al. 2004). Deployment always occurred between 1000 and 1600 h during midtide events, as this is the daily period when fish-grazing activity is likely to be at its highest
(Zemke-White et al. 2002, Fox et al. 2009). After 3.5 h, all algal tethers were retrieved and
reweighed in the laboratory. This tethering procedure was repeated on two different days at
each of the three reef-flat sites across all five regions of Ningaloo Reef (n = 36 treatment and
n = 36 control tethered units per region). The change in fresh weight of each treatment algal
specimen over the deployment time minus the change in fresh weight of its corresponding
paired control provided us with a conservative measure of herbivory by fishes (i.e. uncaged –
caged).
Video analysis: species-specific bite rates
Stationary, high-definition video cameras were used to determine the contribution of
individual herbivorous fish species to the removal of the transplanted Sargassum tethers, a
technique which allows accurate observations of feeding fishes without the potentially
confounding effects of diver presence (Bellwood et al. 2006b, Mantyka and Bellwood 2007b).
For each deployment, two video cameras (Sony-HDR SR12 in underwater housings) were
simultaneously deployed at a site, filming the feeding activity on two randomly selected
uncaged tethers. Video cameras were attached to concrete blocks and placed on the seabed
2-3 m from the selected tether, ensuring the entire macroalgal unit could be viewed in the
frame. During the initial seconds of filming, the camera focal length was calibrated using a
scale bar of known length, which was later transposed onto the viewing monitor where it was
used as a reference scale for analysing the footage. The cameras were set to film
continuously for the entire 3.5 h of assay deployment. This filming procedure yielded 14 h of
footage per site (i.e. 2 x 3.5 h per sample day) and a total of 42 h of footage per region.
To quantify the bite rates of herbivorous fishes, we viewed the entire 210 h of footage and
counted the total number of bites taken by individual fish on each filmed tether. All feeding
fish were identified to species level, with the exception of some initial phase scarids
(Scaridae) and some kyphosids (Kyphosidae). Unidentifiable, dark-coloured juvenile Scarus
species were classified together as Scarus I.P. (initial phase). Likewise, a larger species of
kyphosid (which clearly was not K. vaigiensis, a dominant herbivore at Ningaloo Reef,
Chapter 3) was conservatively classed as Kyphosus sp. (possibly K. sydneyanus and/or K.
bigibbus), as it was difficult to consistently differentiate between the two species using
imagery alone. A ‘bite’ was recorded only if the fish could be seen to apply its jaws to the
tethered alga and close its mouth (following Mantyka and Bellwood 2007b). In those
instances where individual bites could not be counted due to a succession of rapid bites
without a discernable pause, the ‘foray’ was conservatively classed as a single bite event
(Bellwood and Choat 1990). Bites from individuals > 10 cm total length (TL) were tallied into
5 cm size class categories. The total number of bites per fish species was then converted
57
Trophic effects through herbivory at Ningaloo Reef
2011
into a ‘mass-standardised’ bite (bite count × feeding individual’s body mass in kilograms;
following Mantyka and Bellwood 2007b), based on published length-weight relationships
(Kulbicki et al. 2005), and using the midpoint of each size class as the feeding individual’s
-1
TL. Mass-standardised bite counts were then converted to a bite rate (mass bites.min ) by
dividing the count for each species by the period when the algal tether was accessible for
feeding. In those instances where the tether was grazed completely to its basal portions prior
to the 3.5 h period, the available potential feeding time was adjusted accordingly, since the
algae was no longer present for potential feeding by other fishes for the remainder of the
sampling duration. During video analysis, foraging associations were noted, with all bites
from individual fishes being categorised into either group feeding (we define groups
whenever individuals of one or more species forage simultaneously in association with one
another; see Lukoschek and McCormick 2000) or solitary feeding categories.
Distribution of roving herbivorous fishes
The abundances of species within the roving herbivorous fish community (Acanthuridae,
Scaridae, Kyphosidae and Siganidae) from each site were quantified using a series of timed
visual censuses. Underwater visual censuses (UVC) were carried out on snorkel using 10minute timed swims parallel to the contour of the reef-flat, encompassing the area of reef
used for tethering experiments. The distance covered for each 10-minute transect was
measured by trailing an underwater measuring tape which was fixed to the reef at the
starting point of each transect (mean distance of 100.7 m ± 0.5 m). All potentially
herbivorous fish species within a 5 m wide belt-transect, extending from the reef substratum
to the sea surface, were recorded on an underwater slate according to species and nearest
5 cm size class. With the exception of the readily identifiable Chlorurus sordidus (Scaridae),
all other dark coloured initial-phase (I.P.) parrotfishes were recorded as a single group
(Scarus I. P.). Similarly, mixed schools of fish that were likely to be a combination of
Acanthurus blochii and A. grammoptilus (Acanthuridae) were classified together as
Acanthurus spp. due to difficulties in identification. Two UVCs were carried out on each of
the two days that tethers were deployed within each site (n = 12 per region), with a single
UVC conducted immediately prior to the deployment of the tethering experiments, and
another after the deployment period had ceased to minimise disturbance and any potential
feeding deterrence to the herbivorous fish communities. Transect replicates within a site
were separated by a minimum of 20 m each time in order to maximise independence
between census events. Fish counts made from each of the UVCs were converted into
abundance per unit area (individuals 100 m ²) using each individual transect area.
Abundance estimates were then further converted into biomass estimates (grams 100 m ²)
using established length-weight relationships for each species (Kulbicki et al. 2005).
Statistical analyses
Variation in rates of consumption of Sargassum tethers was analysed with a three-factor
ANOVA testing for differences among regions (fixed factor with 5 levels), sites (random
factor with 3 levels nested within each region), and days (random factor with 2 levels nested
within each site and region). Analyses were based on the corrected algal mass removed by
fishes from the six tethered Sargassum replicates deployed each sampling day. Residual
plots and Cochran’s test indicated that the data met the requirements of normality and
homogeneity of variance. Student-Newman-Keuls (SNK) test was used to determine where
the differences lie among regional means.
To examine variation in the overall feeding activity across regions, we pooled the total
number of bites taken by all species for each region and tested for differences using the
same three-factorial ANOVA design previously described. Analyses were based on squareroot transformed bite count data to satisfy the assumptions of ANOVA. For species-specific
mass-standardised bite rates (‘bite rates’ herein) on the filmed Sargassum tethers,
differences among regions and sites and between days were examined using a
permutational multivariate analysis of variance (PERMANOVA, Anderson 2001). The same
58
Trophic effects through herbivory at Ningaloo Reef
2011
three-factor hierarchical design as described previously was also used for PERMANOVA.
Analyses were conducted using Bray-Curtis dissimilarities to emphasise absolute
differences, and were based on square-root transformed data (to reduce the effects of
numerically large values from abundant schooling species; Clarke 1993) with 4999
unrestricted, random permutations. When the fixed region factor was significant, we further
investigated the term through a posteriori pair-wise comparisons using 4999 random
permutations of raw data to obtain Monte Carlo p-values (Anderson 2001). A canonical
analysis of principal coordinates (CAP, Anderson and Robinson 2003, Anderson and Willis
2003) was used as a constrained ordination procedure to visualise and explore patterns in
species-specific bite rates of all species based on the a priori hypothesis of differences
among regions. The bite rates of species likely to be responsible for any observed
differentiation among regions were determined by examining Pearson correlations
(correlation coefficient of r > 0.40) of species-specific bite rates with the canonical axes of
the CAP.
In order to identify the fish species that contributed most strongly to the removal of
Sargassum biomass, we used a simultaneous multiple regression analysis. Bite rates of the
species responsible for >5% of bites in any region (seven species, see Results) were
simultaneously regressed against the biomass removed from the corresponding filmed
Sargassum transplant by fishes. Species which were accountable for <5% of bites in each
region were pooled into higher taxonomic groupings for the analysis. For the analysis,
feeding rate data were square-root transformed to improve normality of the data.
Examination of partial correlations enabled us to identify the individual species (three
species, see Results) that had a significant effect on algal biomass loss. We then tested for
differences in bite rates of these three species across regions, sites and days using
permutational univariate ANOVAs that were based on Euclidean distances and square-root
transformed bite rate data. To examine the influence of group feeding behaviour by these
three browsers on Sargassum removal efficiency, we used a multiple regression analysis
based on the proportion (%) of bites taken by each of the browsers as a group within each
filmed replicate (n=60). To account for the use of proportional data, percentage group
feeding data were arcsine transformed for the analysis.
The biomass of macroalgal browsers, and all roving herbivorous fishes collectively, recorded
during UVCs were compared among regions, sites and days using the same three-factor
hierarchical ANOVA design as described previously. Biomass data were square-root
transformed to meet the requirements of normality and homoscedasticity. Significant
differences in regional means were further explored using the SNK test. Relationships
between the observed herbivorous fish biomass, both collectively and including macroalgal
browsers only, and the reduction in transplanted Sargassum mass, were examined using a
series of Pearson correlation analyses. For the analyses, fish biomass and herbivory data
were averaged at the day within site level, with the biomass of fishes being treated as a
predictor for the corresponding level of herbivory for that same period.
Results
Removal of transplanted macroalgae
Rates of consumption of tethered Sargassum differed among regions of Ningaloo Reef
(Table 4.1), with significantly higher algal mass removed from tethers in Point Cloates (245.6
± 15.2 g) and Gnaraloo Bay (213.2 ± 16.2 g) than in any other region (ranging from 67.1 ±
9.2 g to 137.8 ± 19.7 g) (Figure 4.2a). Rates of consumption of tethered Sargassum also
varied between days, but variation among sites was not statistically significant (Table 4.1).
59
Trophic effects through herbivory at Ningaloo Reef
2011
Species-specific bite rates
Analysis of video footage yielded 32,395 bites (8,969 mass-standardised bites) from 23 fish
species on the 60 filmed Sargassum tethers across the five regions of Ningaloo Reef (Figure
4.2). Although there was no significant variation among regions in the total number of bites
by all fishes pooled (Table 4.2), the distribution of bites among taxa differed considerably
across regions (Figure 4.3b). Seven species, namely Naso unicornis, Kyphosus sp., K.
vaigiensis, Scarus I.P, S. schlegeli, S. ghobban and Siganus doliatus accounted for 95% of
the bites (98% of mass-standardised bites). The bite rates of these seven species explained
approximately 73% of the variation in the loss of biomass from the filmed Sargassum tethers
(Table 4.3). However, partial regressions indicated that, after accounting for autocorrelation
in the data, bite rates of only N. unicornis, Kyphosus sp. and K. vaigiensis were significantly
correlated with the reduction of Sargassum biomass from the filmed tethers (Table 4.3).
The identity of species responsible for the bites observed demonstrated significant variation
among regions, but not among sites within region (Table 4.4a). The a posteriori comparisons
showed that species identity differed for each pair-wise combination of regions (Monte Carlo
p < 0.05), with the exception of Bundegi and Gnaraloo Bay, which did not statistically differ
(Monte Carlo p = 0.1770). Differences in bite rates and feeding assemblages among regions
were highlighted by CAP, which showed a distinct separation of the five regions according to
the identity of the species responsible for the bites (Figure 4.4). Pearson correlations of
species-specific bite rates with the canonical axes of the CAP indicated that 10 species were
correlated (correlation coefficient r > 0.40) with the patterns observed among regions (Figure
4.4). Bite rates of each of the three dominant browsers differed significantly among regions
(Table 4.4b). Point Cloates was characterised by the feeding of Naso unicornis, where it
-1
exhibited significantly higher bite rates (0.83 ± 0.19 kg bites.min ) than in any other region
(Figure 4.3b). The distinction of Gnaraloo Bay and Bundegi from the other regions (Figure
4.4) was largely a result of the significantly higher bite rates of K. vaigiensis in these regions
-1
(0.90 ± 0.19 and 0.67 ± 0.21 kg bites.min respectively; Figure 4.3b). Likewise, the higher
-1
bite rates of Kyphosus sp. was characteristic of Maud (0.72 ± 0.46 kg bites.min ), whereas
Mandu was primarily characterised by higher rates of feeding by Scarus species (Figures
4.3b & 4.4).
Feeding behaviour
The influential browsers, N. unicornis, Kyphosus sp. and K. vaigiensis, were observed
feeding upon the transplanted Sargassum as a part of a monospecific school, a multispecies group, or as solitary individuals. Group browsing associations (mixed, or
monospecific simultaneous feeding) occurred in all regions, except in Mandu where group
feeding by the dominant browsers was never recorded (Figure 4.3d). On average, the
greatest proportions of group feeding by browsers were evident in Gnaraloo Bay (78.5 ± 8.2
% of bites) and Point Cloates (63.6 ± 6.6 % of bites), broadly reflecting regional patterns in
Sargassum removal (cf. Figures 4.3a & d). This variation in feeding behavior by the
macroalgal browsers led to significant positive relationships between the group feeding
activities of these species (collectively and individually) and Sargassum removal efficiency
(Table 4.5).
Relationship between herbivore distribution and macroalgal removal
The collective biomass of roving herbivorous fishes varied significantly across regions (Table
4.6), however, the composition of fishes was dominated by species other than the dominant
macroalgal browsers (Figure 4.3c). Collectively, N. unicornis, Kyphosus sp. and K. vaigiensis
accounted for only 1-7% of herbivorous fish biomass across regions, yet were responsible
for 85-99% of the bite rates on Sargassum being quantified in these regions (except in
Mandu, where browsers were depauperate) (cf. Figures 4.3b & c). Rates of herbivory could
not be predicted based on the distribution of macroalgal browsers in each area, as no
60
Trophic effects through herbivory at Ningaloo Reef
2011
relationship was evident between the consumption of Sargassum and the biomass of
macroalgal browsers collectively, or individually (Table 4.7).
61
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 4.1. Map of the Ningaloo Reef, Western Australia, indicating the regions (open
circles) and sites (filled circles in insets) studied. Dashed polygons depict predominantly
coral reef-flat areas.
62
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 4.2.
The total number of mass-standardised bites, and raw bites, taken by fishes on
the transplanted Sargassum assays across five regions of the Ningaloo Reef.
63
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 4.3.
Variation across the five broad-scale regions of Ningaloo Reef for: A) herbivory
on Sargassum by fishes. Columns represent mean (+ SE) biomass reduction from 36 replicate
Sargassum assays per region. Regional means sharing the same letter do not statistically differ
(SNK post-hoc analysis); B) Mean (+ SE) feeding rate (kg bites.min¯¹) of herbivorous fishes
recorded from video observations (n = 12 filmed replicates per region); C) Mean (+ SE) biomass
(g.100m²) of the dominant macroalgal browsers (Naso unicornis, Kyphosus sp. and K.
vaigiensis) and other roving herbivores (29 species pooled) encompassing the transplant
experiment area (n = 12 UVCs per region); and D) Mean (+ SE) proportion of bites taken by N.
unicornis, Kyphosus sp. and K. vaigiensis whilst part of a feeding group association (n = 12
filmed replicates per region).
64
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 4.4.
Canonical analysis of principal coordinates (CAP) ordination based on BrayCurtis similarities between bite rates (kg bites.min¯¹) of 23 feeding species recorded during
video observations of transplanted Sargassum assays across five regions of the Ningaloo Reef.
Overlaid is a plot of species bite-rate correlations (Pearson correlation coefficient set at > 0.4)
with the canonical axes. Data points represent bite-rates observed from a filmed replicate (n=12
per region). All data were square root transformed.
65
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 4.5.
Rugosity Index and proportional cover of the dominant benthic habitat
categories across each broad-scale region of Ningaloo Reef. Solid line represents mean
Rugosity Index (±SE). Each category within the regional columns represents the mean percent
cover determined from 180 photo-quadrats across three replicate reef-flat sites. The category
‘Others’ includes the pooled percentage contributions to the benthos cover by the categories:
crustose coralline algae, dead coral and damselfish territory.
Figure 4.6.
Non-significant relationship between Sargassum herbivory and the proportion of
naturally occurring macroalgae cover within transplant areas (ANOVA, F = 1.3019, p = 0.3078).
Data points represent mean values for each reef-flat site.
66
Trophic effects through herbivory at Ningaloo Reef
2011
Table 4.1.
Results of ANOVA on biomass removed from Sargassum tethers by fishes,
testing for differences between Regions (50-300 km), among Sites (<1 km) and Days within
each of these spatial scales. Parentheses indicate the levels each of the factors is nested
within. Bold figures indicate significance (p < 0.05).
Source of variation
Region
df
MS
F
p
4
209004.95
10.45
0.001
Site (Region)
10
20003.06
0.60
0.791
Day (Region x Site)
15
33406.83
7.56
< 0.001
150
4420.04
Residual
Table 4.2.
Hierarchical ANOVA on the total number of bites taken by all fishes of the
filmed Sargassum assays, testing for differences between Regions (50-300 km), among Sites
(<1 km) and Days within these spatial scales. Analyses were based on square-root transformed
bite count data. Parentheses indicate the levels each of the factors is nested within and bold
figures indicate significance (p < 0.05).
Source of variation
df
MS
F
p
Region
4
54.71
0.51
0.732
Site (Region)
10
107.97
0.70
0.708
Day (Region x Site)
15
153.50
2.85
0.007
Residual
30
134.85
67
Trophic effects through herbivory at Ningaloo Reef
2011
Table 4.3.
Multiple regression analysis examining the relationship between feeding
species bite rates (kg bites.min¯¹) recorded from video observations and the corresponding
biomass removed from filmed Sargassum assays. Analyses were based on square-root
transformed feeding rate data. Overall model Adjusted R2 = 0.73 (R² = 0.77), F = 16.49,
p < 0.001. Significant (p < 0.05) variables are highlighted in bold text.
Total
bites
Coeffi
cient
SE
Naso unicornis
3299
192.28
25.19
7.63
< 0.001
Kyphosus sp.
565
151.25
22.56
6.70
< 0.001
Kyphosus vaigiensis
13443
157.90
20.19
7.82
< 0.001
Scarus ghobban
647
76.97
104.92
0.73
0.466
Scarus I.P.
2793
-1.96
126.23
0.02
0.987
Scarus schlegeli
7922
42.47
63.52
0.67
0.507
Siganus doliatus
2065
107.95
103.37
1.04
0.301
Other Acanthurids
228
-12.97
196.21
0.07
0.947
Other Scarids
655
-61.49
181.00
0.34
0.735
Other Siganids
777
65.62
162.42
0.40
0.688
t
p
Macroalgae browsers
Grazers
Other
broad
(pooled)
taxa
68
Trophic effects through herbivory at Ningaloo Reef
2011
Table 4.4.
Results of PERMANOVAs examining variation in bite rates (kg bites.min¯¹) of
A) all feeding species, and B) the dominant macroalgal browsers separately, across Regions
(50-300 km), Sites (<1 km) and Days within each of these spatial scales. Parentheses indicate
the levels each of the factors is nested within. The analyses were conducted using Bray-Curtis
dissimilarities (for all feeding species, A) or based on Euclidean distances (macroalgal browsers
individually, B) using square-root transformed bite rate data. P-values were obtained using 4999
unrestricted random permutations. Bold values indicate significance (p< 0.05).
Source of
variation
A)
All species
Naso
unicornis
Kyphosus
vaigiensis
Kyphosus
sp.
MS
Pseudo
-F
p
4
22863.00
9.00
<
0.001
Site
(Region)
10
2539.30
1.00
0.476
Day (Region
x Site)
15
2530.70
2.53
<
0.001
150
998.75
4
1.57
179.23
0.012
Site
(Region)
10
0.01
0.07
1.000
Day (Region
x Site)
15
0.12
16.94
<
0.001
Residual
30
0.01
4
1.68
7.13
0.007
Site
(Region)
10
0.24
1.34
0.287
Day (Region
x Site)
15
0.18
5.97
<
0.001
Residual
30
0.03
4
0.45
2.17
0.011
Site
(Region)
10
0.21
0.75
1.000
Day (Region
x Site)
15
0.28
47.49
0.002
Residual
30
0.01
Region
Residual
B)
df
Region
Region
Region
69
Trophic effects through herbivory at Ningaloo Reef
2011
Table 4.5.
Multiple regression analysis examining the relationship between group feeding
of macroalgal browsers (% bites as a group) and Sargassum removed from filmed replicates.
2
Percentage feeding data were arcsine transformed for the analysis. Overall model Adjusted R
= 0.65 (Multiple-R² = 0.66), F = 36.6, p < 0.001. Significant (p < 0.05) variables are highlighted
in bold text.
Species
Coefficient
SE
t
p
Naso unicornis
125.10
19.99
6.25
< 0.001
Kyphosus sp.
180.55
31.08
5.81
< 0.001
Kyphosus vaigiensis
118.45
17.05
6.94
< 0.001
Table 4.6.
Hierarchical ANOVA on total herbivorous fish (all roving species pooled)
-2
biomass (g.100m ), testing for differences between Regions (50-300 km), among Sites (<1 km)
and Days within each of these spatial scales. Analyses were based on square-root transformed
biomass data. Parentheses indicate the levels each of the factors is nested within and bold
figures indicate significance (p < 0.05).
Source of variation
df
MS
F
p
Region
4
2038.80
3.78
0.040
Site (Region)
10
539.38
1.65
0.184
Day (Region x Site)
15
326.99
2.42
0.019
Residual
30
134.85
70
Trophic effects through herbivory at Ningaloo Reef
2011
Table 4.7.
Relationship between roving herbivorous fish biomass (g.100m²) and the
biomass removed from transplanted Sargassum by fishes across all regions of the Ningaloo
Reef. Fish biomass data were square-root transformed and bold figures indicate significance
(p<0.05). The regional mean (±SE) biomass for each group of fishes is also given.
Species
Mean
regional
biomass
2
100m )
All roving herbivorous fishes (32
species pooled)
Correlation
coefficient
R
2211.67
(±282.71)
-0.03
0.00
0.887
Macroalgal browsers (3 species
pooled)
115.23
(±21.42)
0.19
0.03
0.328
Naso unicornis
100.81
(±20.35)
0.24
0.06
0.201
Kyphosus vaigiensis
14.42 (±6.10)
-0.03
0.00
0.880
Kyphosus sp.
(g.
2
p
Species not observed during any UVC
Scarus ghobban
95.35 (±20.48)
-0.37
0.13
0.047
Scarus schlegeli
48.56 (±7.78)
-0.23
0.05
0.228
Scarus I.P.
129.63
(±24.60)
-0.23
0.05
0.225
Siganus doliatus
38.86 (±12.82)
-0.30
0.09
0.113
71
Trophic effects through herbivory at Ningaloo Reef
2011
Table 4.8.
Multiple one-way ANOVAs examining variation in the major benthic categories
among regions. All benthic cover data was square root arcsine (%) transformed and rugosity
data were arsine (proportion) transformed. Bold figures indicate significant (p < 0.05) values.
Benthic category
Source of variation
df
MS
F
p
Live coral
Region
4
173.30
3.12
0.065
Residual
10
Region
4
405.09
25.84
<0.001
Residual
10
Region
4
102.26
6.16
0.009
Residual
10
Region
4
423.86
26.12
<0.001
Residual
10
Region
4
0.06
2.19
0.142
Residual
10
EAM
Macroalgae
Sand
Rugosity Index
72
Trophic effects through herbivory at Ningaloo Reef
2011
Discussion
Our study demonstrated a high level of variability in rates of herbivory by fishes, and in the
identity of the main herbivores, across the entire spatial extent encompassed by a fringing
coral-reef ecosystem, the Ningaloo Reef (ca. 300 km). We also identified that it is the
variation in feeding of a few species within multi-species feeding assemblages that drives the
observed patterns of macroalgal removal. Despite the presence of at least 32 roving
herbivorous fish species (as determined through UVCs in this study), consumption of
tethered Sargassum was largely due to three fish species, namely Naso unicornis
(Acanthuridae), Kyphosus sp. and K. vaigiensis (Kyphosidae). The dominance of N.
unicornis and kyphosids in this west-continental coral-reef system supports and extends on
findings in Ningaloo Reef (Chapter 3) and previous findings from inner- and mid-shelf GBR,
which have also identified that a subset of the species present are usually responsible for a
disproportionate amount of consumption, with the identity of these consumers varying from
place to place (Bellwood et al. 2006b, Mantyka and Bellwood 2007a, Fox and Bellwood
2008, Cvitanovic and Bellwood 2009, Hoey and Bellwood 2009, Bennett and Bellwood
2011).
Peterson et al. (1998) hypothesised that when there are several species performing similar
functional roles within a single system, they may provide a mutual reinforcement to one
another that contributes to the resilience to disturbances. In the present study, three species,
Naso unicornis, Kyphosus sp. and K. vaigiensis demonstrated the capacity to consume
tethered Sargassum across different regions of Ningaloo Reef, indicating the potential for
some, albeit limited, functional redundancy within this system. This disproportionate role of a
select few species within the herbivorous guild to remove this dominant alga appears to be a
constant feature elsewhere, regardless of the spatial resolution being examined (cf.
Cvitanovic and Bellwood 2009, Hoey and Bellwood 2009, Bennett and Bellwood 2011). The
stability of these species is, therefore, likely to be critical to the resilience of coral-reef
systems of Australia, and perhaps elsewhere. The failure of many reefs to recover following
perturbations, while undergoing a transition from coral- to macroalgal-dominated states, may
be partially due to the absence of, or limited functional redundancy within, such critical
functional groups (Bellwood et al. 2004).
Other roving herbivorous species played only a minor role in the removal of macroalgal
biomass, despite high bite counts by several species, including the parrotfishes Scarus I.P.,
S. schlegeli and S. ghobban, and the rabbitfish Siganus doliatus. We observed these fishes
taking small, rapid bites when feeding (Michael unpubl. data), resulting in little apparent
contribution to the algal biomass loss. These findings are consistent with other studies,
which have also described the functional inability of these groups of grazing fishes to rapidly
remove erect macroalgae (e.g. Bellwood et al. 2006b, Fox and Bellwood 2008, Fox et al.
2009). In contrast, the success of N. unicornis and kyphosids in consuming tethered
Sargassum is not surprising, since these fishes are among a small group of coral-reef fishes
that are morphologically and physiologically capable of consuming some fucoid brown
macroalgae (Clements and Choat 1997, Choat et al. 2004, Crossman et al. 2005).
The variable role of fishes and their relative importance in mediating coral-reef algal
assemblages is recognised to be dynamic, but the variability in our study raises the question
as to why the identity of the main herbivorous species, and the subsequent levels of
quantified herbivory differ markedly within the same habitat types across broad-scale regions
of a single coral-reef system. Such variation may partially be the result of variation in
macroalgal browser distribution, the relative abundance, susceptibility and palatability of the
tethered Sargassum, differences in structural complexity of the transplant habitat and
differences in browser feeding behaviour among regions. These options are discussed
below.
The removal rate of Sargassum was not related to the biomass of macroalgal browsers
observed in UVC, both individually and collectively. One of the main consumers of tethered
73
Trophic effects through herbivory at Ningaloo Reef
2011
Sargassum, Kyphosus sp., was never detected during visual censuses, yet in the absence of
divers (as shown by video) they fed in groups of greater than 15 individuals and had a
dramatic impact on the Sargassum tethers. Conversely, although N. unicornis was observed
on UVC in Bundegi and Mandu, it did not feed in these regions at all. Such difficulties in
predicting ecosystem function based on presence has been highlighted in similar
observational-based studies, which have also failed to detect the key drivers of macroalgal
herbivory based on underwater visual observations alone (Fox and Bellwood 2008,
Cvitanovic and Bellwood 2009, Hoey and Bellwood 2009). Importantly, these results
highlight that the species critical to the functioning and resilience of reef systems against a
change in macroalgal abundance may go unnoticed through the single-handed use of
underwater visual observations of fish communities. This increased awareness of observerbased limitations has reinforced the importance of the recent novel direction in herbivory
studies which aim to directly quantify species-specific impacts, rather than the use of
inference (e.g. Bellwood et al. 2006b, Mantyka and Bellwood 2007b, Burkepile and Hay
2008, Fox and Bellwood 2008, Cvitanovic and Bellwood 2009, Hoey and Bellwood 2009,
Bennett and Bellwood 2011, Chapter 3).
Variation in consumption rates of Sargassum tethers may be the result of regional
differences in the abundance, susceptibility and palatability of the transplanted macroalgae.
Removal rates of the tethers by fishes may be partially influenced by the abundance and
relative palatability of resident algal communities within the local area (Cvitanovic and Hoey
2010, Hoey and Bellwood 2010a), as well as the density of the tethered algae itself (Hoey
and Bellwood 2011). Regional cover of resident macroalgae was significantly higher in Maud
(15 ± 1.5%) (Figure 4.5 & Table 4.6), a pattern driven by a seasonal patchy understory
growth of predominantly Turbinaria which is consumed by N. unicornis (Choat et al. 2002)
and K. vaigiensis (Clements and Choat 1997) elsewhere. Despite this availability of an
alternative food source, there was no distinct relationship between the abundance of
macroalgae and rates of Sargassum removal in this study (Figure 4.6). In terms of
palatability, we minimised potential variation of this trait by using a single algal species in a
similar condition (S. myriocystum without visible large epiphytes) for all tethers. Although
intra-specific variation in nutritional traits and/or chemical defences among local populations
is possible and has been reported to influence herbivory by invertebrates in a temperate
system (Taylor et al. 2003), secondary metabolite concentrations are relatively lower in
tropical species of Sargassum (Steinberg 1986), often making them ineffective in deterring
many tropical herbivorous fishes (Steinberg and Paul 1990, Steinberg et al. 1991).
Moreover, the conclusion that variation in chemical or physical deterrence is unlikely to
explain variation in grazing rates is supported by the fact that the Sargassum used at both
Gnaraloo Bay and Maud was collected from the same meadow, yet its consumption rates
differed significantly between these two regions.
Feeding rates by herbivorous fishes have been shown to be influenced by the provision of
benthic structural complexity (Fox and Bellwood 2007, Vergés et al. Chapter 2) and have
been suggested to partly explain the different macroalgal consumption rates by browsers
across regions within the GBR (Bennett and Bellwood In press). Macrophytes growing in
refuge areas from intense fish grazing (such as the Sargassum collected from unstructured
lagoon habitats in this study) are generally poorly defended against herbivory, and are
rapidly consumed when moved to a nearby structured habitat with a higher abundance of
herbivorous fishes (Hay 1981a, Lewis 1986). However, in our study, we tethered Sargassum
to similarly structured reef-flat substrata in all regions (Figure 4.5), thereby minimising any
regional variation in refuge availability or accessibility to the macroalgal tethers by fishes.
An alternative explanation for the variation in feeding rates may relate to differences in
feeding behaviour of the browsing species observed in the present study. We observed a
large portion of the bites by the dominant macroalgal browsers, N. unicornis, Kyphosus sp.
and K. vaigiensis, occurring whilst part of monospecific or multi-species foraging
associations, and therefore, detected a significant positive relationship between this group
feeding activity and Sargassum removal efficiency. Such group foraging may enhance the
ability of some individuals to locate and consume resources more rapidly than when alone,
with successful foragers attracting other group members through behavioural cues
74
Trophic effects through herbivory at Ningaloo Reef
2011
associated with successful foraging (Baird et al. 1991, Lukoschek and McCormick 2000).
This general foraging theory appears to be consistent with the feeding behaviour of fishes
observed in our study, where individuals often made several passes at the filmed tethered
Sargassum before being joined by conspecifics for group feeding. Group feeding appears to
be a behaviour exhibited by macroalgal browsers elsewhere (e.g. Siganus canaliculatus, Fox
and Bellwood 2008; N. unicornis, Hoey and Bellwood 2009), as it probably serves as
predator protection (Ogden and Lobel 1978) or a strategy to overcome the territorial defence
of algal patches by territory holding species (Robertson et al. 1979, Foster 1985). Several
authors have described the greater foraging rates of fishes when feeding in a group
compared to when alone (Robertson et al. 1979, Foster 1985, Reinthal and Lewis 1986, Wolf
1987), however, to the best of our knowledge, this study represents the first attempt to
directly quantify the influence of group feeding by fishes on Sargassum removal efficiency in
a coral-reef system. In our study, this feeding behaviour appeared to have influenced the
feeding interactions within and among species and, therefore, the variability in speciesspecific feeding rates and herbivory being quantified. For example, the Sargassum mass
loss was nearly two and a half times greater in Gnaraloo Bay than in Bundegi; yet bite rates
of the dominant browser K. vaigiensis did not differ between the two regions. In this case, the
kyphosids in Bundegi took more than half the total bites whilst feeding as individuals,
whereas, schools of up to 15 simultaneously feeding fishes took on average 80% of the bites
in Gnaraloo Bay. An alternative, that is not mutually exclusive, is that variation in territorial
behaviour may also explain some of the variation in algal biomass loss. In some sites,
aggressive territorial behaviour was displayed by juvenile K. vaigiensis as they often chased
other herbivorous fishes from the filmed tethers during feeding (Michael unpubl. data). Such
territory-holding behaviour has been reported for kyphosids elsewhere (Hamilton and Dill
2003), and further highlights the need to consider the behavioural aspects of species
interactions when distinguishing the potential impact of a species on ecosystem processes.
The extent to which these feeding behaviours exclude other fishes or alter efficiency in
macroalgal consumption is partially unclear, but is likely to explain some of the variability that
occurs across space and time in this study.
Typically, nearshore coral reefs are faced with the most imminent predisposition to change
due to their close proximity to increasing human use and terrestrial influences (e.g.
McCulloch et al. 2003). However, the Ningaloo Reef represents a unique coral-reef system,
which unlike many of its counterparts, is relatively unaffected by anthropogenic pressures
and receives little input from terrestrial sources (Cassata and Collins 2008). The results of
our study, therefore, may provide a useful baseline for our understanding of the potential role
of macroalgal browsers in a relatively intact nearshore coral-reef system. Given the
unprecedented worldwide decline in coral-reef health in recent decades (Gardner et al. 2003,
Pandolfi et al. 2005, Bruno and Selig 2007), and subsequent urgent calls for processoriented research and management (Hughes et al. 2003, 2005, Folke et al. 2004),
understanding variation in mechanisms such as herbivory that promote the resilience of coral
reefs is essential. We are currently moving beyond just recognising the importance of the
entire herbivorous guild in shaping benthic communities, to acknowledging the speciesspecific quantitative nature of these algal-herbivore interactions. In this study, the use of
remote video cameras has provided an insight into the highly variable patterns of macroalgal
herbivory and the key drivers that are likely to play an integral role in maintaining a healthy
balance between coral and macroalgae across multiple spatial scales of the Ningaloo Reef.
75
Trophic effects through herbivory at Ningaloo Reef
2011
CHAPTER 5. Variability in the food sources of herbivorous
invertebrates and fishes in a coral-reef system: a stable
isotope approach
Glenn Hyndes, Adriana Vergés and Mathew Vanderklift
Introduction
Herbivores in coral-reef systems play an integral role in removing macroalgae that can
otherwise outcompete corals (Hughes et al. 2003, Bellwood et al. 2004). Altering the
herbivore-algae balance of interactions through disturbance, such as overfishing (Jackson et
al. 2001) or disease (Hughes 1994), can result in a shift from a coral- to macroalgaedominated state (Bellwood et al. 2006c, Hughes et al. 2007 ). The high diversity of
herbivores, particularly fish, in coral-reef systems (Clements et al. 2009) has produced
considerable debate regarding functional redundancy within this group in terms of
maintaining the balance between corals and macroalgae (Bellwood et al. 2003, Fox et al.
2009).
The functional roles within “nominally” herbivorous fishes can differ markedly. Roving
herbivorous fishes can be broadly classified into two functional groups: grazers and
browsers (Hoey & Bellwood 2010a). The grazers include scraping and excavating parrotfish
(Labridae), which feed on the epilithic algal matrix (EAM) and algal turf (sensu Wilson et al.
2003), while browsers (mostly members of Kyphosidae and some Acanthuridae) remove
erect, tough macroalgae from the reef (Bellwood et al. 2006a). This functional group
approach has provided insights into the role of nominally herbivorous fishes in maintaining
coral-reef ecosystems (Hughes et al. 2003, Bellwood et al. 2004). However, it has been
argued that a greater focus should be placed at the species level (Clements et al. 2009).
Different species may share similar morphologies and feeding behaviours, but they can
exhibit differences in their nutrient intake and assimilation (Bellwood et al. 2003, Crossman
et al. 2005) and grazing impacts may also differ among species (Fox & Bellwood 2007).
Thus, there is a need to understand the relative roles of herbivores at the species level to
explain interactions between consumers and their food sources within coral-reef
ecosystems.
Knowledge on the role of “nominally” herbivorous fishes in coral-reef systems has generally
been gained through two distinct approaches. Firstly, an understanding of herbivore nutrition
has been gained through examination of gut contents and biomarker (e.g. lipids,
carbohydrates and amino acids) composition of the digestive tract and tissue of fishes (e.g.
Wilson et al. 2001, Choat et al. 2002, Choat et al. 2004, Crossman et al. 2005). The use of
lipids particularly has provided insights into the importance of detritus and zooplankton in the
diets of a range of “nominally” herbivorous fishes (Wilson et al. 2001, Crossman et al. 2005).
Secondly, an understanding of the impact of grazing on the algal assemblages has been
gained through macroalgal bioassays experiments ( Fox & Bellwood 2007, Hoey & Bellwood
2010a). This approach has allowed differences in grazing rates on macroalgae across
species, regions and habitats to be determined (Fox & Bellwood 2007, Hoey & Bellwood
2010a, Vergés et al. 2011, Bennett & Bellwood 2011), regardless of whether nutrients from
the algae are assimilated by the grazers and incorporated into the food web. Surprisingly,
few studies have attempted to provide an understanding of the integration of food sources by
these herbivores.
Stable isotopes provide an alternative and complementary tool to characterise food webs in
marine ecosystems, and have been valuable in examining food-web interactions in several
marine environments (e.g. Moncreiff & Sullivan 2001, Adin & Riera 2003, Melville & Connolly
2003, Hyndes & Lavery 2005). In tropical environments, stable isotopes have provided an
understanding of food web interactions in a mosaic of habitats within coral-reef landscapes
76
Trophic effects through herbivory at Ningaloo Reef
2011
(e.g. Yamamuro et al. 1995, Nagelkerken et al. 2006, Nyunja et al. 2009) as well as
providing insights on specific trophic level and ontogenetic shifts in diets of coral-reef fishes
(e.g. de la Moriniere et al. 2003, Carassou et al. 2008, Nakamura et al. 2008, Greenwood et
al. 2010). However, our understanding of the variability in food sources among assimilated
by fishes broad-scale regions or coral habitats is limited. Grazing rates by herbivorous
species in coral-reef systems can vary over large spatial scales (Bennett & Bellwood 2011,
Vergés et al. In review), but these algal-assay studies provide little insight into the variation
of food-source integration into the coral-reef food webs. Yet, the importance of various food
sources has been shown through stable isotopes to vary over small (e.g. habitat) and large
(regional scales) for a range of consumers in other marine systems (Wing et al. 2008,
Vanderklift & Wernberg 2010).
In this study, we hypothesised that the food sources would vary among species within the
two functional groups of nominally herbivorous fish species, namely scrapers and browsers,
as well as herbivorous invertebrate species in a coral-reef system. Furthermore, we
predicted that the food sources would vary among regions (over a spatial scale of 10s
kilometres) and among habitats (over a cross-shelf spatial scale of 100s metres) within each
15
13
species. We have used dual stable isotopes (δ N and δ C) to infer dietary sifts among and
within species at Ningaloo Reef on the west coast of Australia, and in doing so, examined
15
13
the spatial shifts in δ N and δ C for a range of potential dietary sources (autotrophs) and
herbivorous fish and invertebrate species. This has allowed us to determine, through mixing
model analyses of stable isotopes, whether the assimilation of food sources differ among
and within herbivorous species.
Materials and methods
Study area and sample collection
This study was conducted on the Ningaloo Reef, a fringing coral-reef system extending ~290
km along the north-west coast of Western Australia. Regional comparisons within reef-flat
habitat were made among three Sanctuary Zones where no fishing activities are allowed
(Bundegi 21°51’S, 114°10’E; Mandu 22°05’S, 113°52’E; Maud 23°05’S, 113°44’E;), while
habitat comparisons were made among outer-reef, reef-flat and lagoon habitats within the
Mandu Sanctuary Zone (Figure 5.1). All sampling was undertaken at depths between 1 and
6 m in each habitat/region over two months (July - September 2008) to minimise temporal
variations in isotopic signatures (Owens 1987). The coral-dominated reef-flat habitat was
chosen for regional comparisons because this habitat displays the highest levels of herbivory
and herbivore biomass (Chapter 2).
Within each region/habitat, 3-5 replicate samples of benthic primary producers and
herbivorous invertebrates were haphazardly collected by hand by divers over distances of
metres to 100s of metres. Samples of Epilythic Algal Matrix (EAM) were obtained by
collecting boulders and pieces of dead coral and gently brushing off all loose sediment and
detritus particles with a soft brush. These aqueous EAM samples were filtered through a 125
µm sieve and allowed to settle for 1-2 hours before excess water was decanted. In each
region/habitat, 3-5 individuals of two gastropod species and one echinoderm species were
collected by hand, and14 nominally herbivorous fish species were collected by spear fishing.
Since Kyphosidae species were often difficult to distinguish, they have been pooled for this
study. Muscle tissue was taken from the foot of gastropods, the Aristotle’s lantern of sea
urchins, and the dorsal body of fish. Samples were then frozen immediately after collection
and processing and stored at -20°C until laboratory analysis. Prior to analysis, macrophytes
were rinsed with seawater, and cleaned of epiphytes where necessary.
Sampling resulted in three species of invertebrates and 14 species of nominally herbivorous
fishes, including 8 species designated as scrapers and six as browsers (Table 5.1).
77
Trophic effects through herbivory at Ningaloo Reef
2011
Stable isotope analysis
All samples were freeze-dried, homogenized to a fine powder using a ball mill and weighed
into tin capsules. For those samples requiring removal of inorganic carbon (e.g. calcareous
algae, EAM), subsamples were weighed into silver capsules and acid-treated by adding
13
drops of 1N HCl until effervescence ceased (Boutton 1991) before δ C analyses. All
samples were analysed for stable isotopes by firstly combusting samples in an elemental
analyser (ANCA-GSL, Europa, Crewe, United Kingdom) and purifying them by gas
chromatography. Nitrogen and carbon elemental composition and stable isotope ratios were
then determined using continuous flow isotope ratio mass spectrometry (20-20 IRMS,
Europa, Crewe, United Kingdom). For calibration, samples were interspersed with reference
materials of known elemental composition and stable isotope ratios. Reference had
previously been calibrated against International Atomic Energy Agency (IAEA) or National
Institute of Standards and Technology (NIST) reference materials with a precision (1 SD
from ~10 samples) of < 0.1‰. Results are reported relative to Vienna PeeDee Belemnite (V13
15
PDB) for δ C and atmospheric N2 for δ N, and expressed in δ notation as: δX (‰) =
13
15
13
12
15
14
((Rsample/Rstandard) – 1) × 1000, where X = C or N, and R = C: C or N: N.
Data analyses
Statistical and mixing modelling analyses were conducted for each species/group that was
represented in at least 2 regions or 2 habitats, and by at least 3 replicate samples from each
region or habitat. One-way univariate PERMANOVA was used to test for the magnitude and
13
15
significance of variation among regions or habitats in δ C and δ N based on Euclidean
distance-based linear models with significance tests by permutation using PERMANOVA+
add-on package for PRIMER v6 (Anderson 2001, McArdle & Anderson 2001, Clarke &
Gorley 2006, Anderson et al. 2008b). Euclidean distance measures, in these univariate
cases, yield estimates of sums of squares equivalent to parametric ANOVA, but the use of
permutation allows for significance to be tested without the assumption of normality.
PERMDISP (equivalent to Levene’s test for heterogeneity of variances in the univariate tests
(Anderson et al. 2008b) was used to test for homogeneity of dispersion. We have focused on
the relative magnitude of effect (the variance component of each factor, divided by the sum
of all variance components), which unlike P values, does not depend directly on the degrees
of freedom (Graham & Edwards 2001). Any negative variance components were set to zero
following the pooling procedure outlined by Graham and Edwards (2001).
15
13
Mixing model analyses were conducted on dual isotopes (δ N and δ C) using MixSIR (V
1.0.4), a Bayesian stable isotope mixing model incorporating variability (Moore and
Semmens 2008) to examine the potential dietary contributions of herbivorous invertebrate
and fish species in each region for the reef-flat habitat and in each habitat at Mandu.
Sources for each consumer species were selected from the list used for statistical analyses
(above), based on whether they were relatively abundant in each region/habitat (Vanderklift
15
13
and Babcock, unpublished data). Where the mean δ N and δ C for species of sources
within a functional algal group (e.g. large brown macroalgae) were within 1‰ in each
region/habitat, the replicate data were pooled for mixing model analyses, as they were
considered to be similar and pooling would reduce any ambiguity associated with stable
isotope results. Based on published literature, discrimination levels (Δ) of 2.4‰ (±1.8 SD)
15
13
and 0.6‰ (±0.2 SD) were used for δ N (McCutchan et al. 2003) and δ C (DeNiro & Epstein
1981), respectively, in the mixing model for invertebrates, and 4.8‰ (±1.3 SD) and 1.8‰
15
13
(±0.2 SD) for δ N (Mill et al. 2007) and δ C (Caut et al. 2009), respectively, for nominally
herbivorous fish. Sufficient iterations were carried out to ensure at least 1000 posterior draws
in the final output for each mixing model run. The distributions of feasible solutions are
th
th
th
th
th
presented by providing the 5 , 25 , 50 (median), 75 and 95 percentiles for each potential
source contribution.
78
Trophic effects through herbivory at Ningaloo Reef
2011
Results
Species and regional comparisons
15
All species of brown algae exhibited relatively low variability in δ N, as highlighted by the
variance components, though variability was greatest at the region level particularly for
Turbinaria ornata (Table 5.2, Figure 5.2). In comparison, Sargassum oligocystum showed far
13
higher variability for δ C, which was mainly explained by within-region differences, while
Lobophora variegata showed greater regional variation in this isotope. Again, the variance
15
components of δ N were low for red algae, but were proportionately higher across regions in
the case of foliose alga Hypnea pannosa and crustose coralline algae, and within region for
the foliose algae Acanthophora spicifera and Portiera hornemanii and articulated coralline
13
15
algae (Table 5.2, Figure 5.2). Variability of δ C was often greater than that of δ N for red
algae, with the greatest variation exhibited across regions for A. spicifera, articulated
coralline algae and crustose coralline algae. In comparison, all the variation was explained
15
by within-region differences for H. pannosa and P. hornemanii. Variation in both δ N and
13
δ C was relatively small for the seagrass Halophila sp., but was apportioned mainly by
13
region and within region for the respective isotopes. Variance components of δ C were high
for EAM, which exhibited greatest variance within region, while cyanobacterial matt exhibited
15
the highest δ N variance among regions (Table 5.2, Figure 5.2).
All three herbivorous invertebrate species exhibited relatively low variance components for
15
δ N (Table 4.2). Variability was higher within regions for this isotope in the gastropod Tectus
pyramis and the urchin Echinometra mathaei (Table 5.2, Figure 5.2). Variance was far
13
15
greater in δ C than δ N for all invertebrate species, with the two gastropod species
13
displaying greater variance than the urchin. Tectus pyramis exhibited greatest δ C variation
between regions (Table 5.2, Figure 5.2).
15
Similar to invertebrates, the variance components of δ N were generally low for all fish
15
species. However, Kyphosus spp. displayed far greater variation in δ N, which was
explained by within region differences (Table 5.2, Figure 5.2). In comparison, variation was
13
far higher in terms of δ C. The scrapers Acanthurus dussumieri, Acanthurus triostegus,
Ctenochaetus striatus, Scarus frenatus and S schegeli all showed high within region
13
variability in δ C. Similarly. the browsing species Naso unicornis (Acantharidae), Kyphosus
spp. (Kyphosidae) and Siganus argenteus (Siganidae) showed high variability within regions
13
for δ C, with Kyphosus spp. exhibiting particularly high variance (6.67).
Mixing model outputs showed that the contributions of the different potential dietary sources
varied considerably among regions in the reef–flat habitat for a range of consumer species.
Based on MixSIR outputs, the brown alga Lobophora variegata (pooled with either Turbinaria
ornata or Dictyota sp.) contributed the most to the diets of Turbo sp. at Mandu and Maud,
whereas crustose coralline algae made the greatest dietary contributions to this gastropod
species at Bundegi (Figure 5.3). A range of other potential sources also contributed to the
diet of this species and the proportional contribution of those sources varied among regions.
In comparison, Lobophora variegata pooled with T. ornata, and to a lesser extent EAM,
contributed the most to the diets of Tectus pyramis at Mandu, but the main dietary sources
differed for this gastropod species in the other two regions (Figure 5.3). For the urchin
Echinometra mathaei, L. variegata (on its own or pooled with T. ornata,) as well as EAM
made the greatest contributions to its diets at both regions it was collected (Mandu and
Maud).
Despite scarids being categorised as scrapers (Table 5.1), they showed high variability in
their dietary sources (including macroalgae) among species and even within species (across
regions). For Chlorurus sordidus, EAM was an important dietary source at Bundegi, while
Lobophora variegata (pooled with T. ornata or Dictyota sp.) also contributed to its diet at
79
Trophic effects through herbivory at Ningaloo Reef
2011
Mandu and Maud (Figure 5.4). Similarly, EAM was an important dietary source for Scarus
schegeli at Bundegi, but red algae also contributed to its diet in this region. In comparison,
macroalgae contributed mainly to the diet of this species at the other two regions. EAM also
contributed to the diet of S. ghobban at Mandu, but its contribution was negligible at the
other two regions where its diet comprised a mixture of macroalgae. In comparison, S.
frenatus exhibited a mixture of dietary sources at Mandu and Maud, the two regions in which
it was collected. There was considerable variability in the dietary sources across regions and
within regions for each of these three scraper species (Figure 5.4).
The scrapers in Acantharidae, Acanthurus dussumieri, Acanthurus triostegus and
Ctenochaetus striatus, showed similar high variability in their dietary sources across regions,
as well as among species within a region. For example, the mixing models for A. dussumieri
and A. triostegus identified high contributions of the foliose red alga Hypnea pannosa at
Maud, and either P. hornemanii or A. spicifera at Bundegi, but foliose red algae made only
small contributions to the diets of these species at Mandu where the diet was shown to be
mixed (Figure 5.5). It should be noted that the contributions of these algal species to the
diets were highly variable for each species and region, particularly at Bundegi. Ctenochaetus
striatus was the only acanthurid species to show high contributions of EAM, and this was
restricted to Bundegi.
In the case of browsers, both Naso unicornis and Kyphosus spp. almost exclusively
assimilated nutrients from Lobophora variegata (pooled with T. ornata or Dictyota sp.) at
Bundegi and Mandu (Figure 5.5). The latter species also ingested T. ornata to some degree
(median = 0.25) at Maud. The foliose red alga A. spicifera made greater but variable
contributions to both N. unicornis and Kyphosus spp in this region. In comparison, the diet of
Z. scopas, another browsing species in Acanthuridae, consisted predominantly of foliose red
algae (either H. pannosa or A. spicifera) within both regions it was caught, although their
contributions were variable and the brown algae L. variegata and Dictyota sp. could also
make high contributions at Maud. For browsers in Siganidae, namely S. argenteus and S.
doliatus, the foliose red algae (either H. pannosa or A. specifera) made high dietary
contributions at Bundegi and Maud, but both EAM and the brown alga Sargassum
oligocystum made relatively high contributions to the former species at Mandu (Figure 5.4).
Species and habitat comparisons
Similar to regional comparisons, macroalgae displayed relatively low variance components
15
for δ N in habitat comparisons at Mandu (Table 5.3). In proportionate terms, the brown alga
Lobophora variagata as well as articulated coralline algae and Portiera hornemanii exhibited
15
high variability in δ N across habitats (Table 4.3, Figure 5.6). Variance components were
13
15
generally higher for δ C than δ N for these algae (Table 5.3). Crustose coralline algae
13
showed greatest variation within habitats for δ C, while articulated coralline algae varied
mainly across habitats (Table 5.3, Figure 5.6). In the case of EAM, variability was higher in
13
δ C, and this occurred within habitat.
The variance components of δ13C were far higher than those of δ15N for the invertebrates
13
and fishes (Table 5.3). Variability in δ C was explained mainly by habitat differences for T.
pyramis, Turbo sp. and E. mathaei, and for the scrapers A. dussumieri, A. triostegus and C.
striatus (Table 5.3, Figure 5.6). In comparison, another scraper, Scarus frenatus, showed
greater variability within habitats, which was similar to the browsing species N. unicornus
and Kyphosus spp. (Table 5.3, Figure 5.6). Indeed, the variance of 13.19 for Kyphosus spp.
within habitat was exceptionally high.
Similar to regional comparisons, the contributions of the different potential dietary sources
varied considerably among habitats for consumer species at Mandu as shown by mixing
model outputs. EAM and the brown alga Lobophora variegata made relatively high
contributions (median >0.4) to the diet of the urchin E. mathaei at both lagoon and reef-flat
habitats (Figure 5.7), although the range of contributions was high (5 percentile <0.2, 95
percentile >0.7). In contrast, a mixture of all sources contributed to the diet of this species in
80
Trophic effects through herbivory at Ningaloo Reef
2011
the outer-reef habitat. EAM and L. variegata made similarly high contributions to gastropod
T. pyramis in the lagoon habitat (median = 0.25 and 0.50 respectively), but their
contributions varied widely in this habitat (Figure 5.7). EAM made lower contributions to the
diet of this species in the reef flat habitat, while articulated coralline algae and the brown
algae Sargassum oliocystum and L. variegata all made relatively high contributions in the
outer-reef habitat. Turbo sp. exhibited similar dietary sources to T. pyramis in the outer-reef
habitat, but its sources varied between this and the reef–flat habitat at Mandu (Figure 5.7).
In terms of nominally herbivorous fishes, a range of sources contributed to the diets of the
scrapers, the including S. ghobban and S. frenatus, as well as the acantharids, Acanthurus
blochii, Acanthurus dussumieri, Acanthurus triostegus and Ctenochaetus striatus (Figures
5.7 and 5.8). Macroalgae made the greatest contributions to these scrapers, but the
contributions of algal species varied among habitats and between fish species, and also
varied within habitat and species. For example, the foliose red alga P. hormenanii made high
contributions to A. blochii and A. dussumieri in the lagoon habitat (median >0.8), although its
contributions were highly variable for the former species (Figure 5.8). In contrast, S.
oligocystum made high contributions to the diet of A. blochii in the reef-flat habitat, albeit
variable. For A. dussumieri, both P. hormenanii and S. oligocystum made high contributions
in the reef-flat habitat, while S. oliocystum almost dominated its diet in the outer reef.
Similarly, the sources to the diets of A. triostegus and C. striatus varied among habitats.
EAM made relatively low contributions to the diets of most scrapers in all habitats (Figures
5.7 and 5.8).
For the browsers N. unicornis and Kyphosus spp., the brown alga Lobophora variegata
dominated their diets in both reef-flat and outer-reef habitats (Figure 5.8). This alga made far
lower, but more variable, contributions to the diet of N. unicornus in the lagoon habitat, while
another brown alga, S. oligocystum, made far greater contributions to the diet of this browser
in this habitat.
81
Trophic effects through herbivory at Ningaloo Reef
2011
Table 5.1 – Nominally herbivorous fish species collected at Ningaloo Reef, with their
designation to functional groups based on broad dietary data from the literature and web
sources.
Species
Functi
onal
group
Diet
Reference
Acanthuridae
Acanthurus blochii
Acanthurus dussumieri
Acanthurus triostegus
Ctenochaetus striatus
Naso unicornis
Scraper
Scraper
Scraper
Scraper
Browser
EAM
EAM
EAM
EAM
Large macroalgae
Zebrasoma scopas
Browser
Thallate
&
filamentous algae
Cvitanovic et al. (2007)
Cvitanovic et al. (2007)
Cvitanovic et al. (2007)
Froese and Pauly (2011)
Choat et al. (2002); Hoey &
Bellwood (2009)
Choat et al. (2002)
Browser
Macroalgae
Clements and Choat (1997)
Browser
Large macroalgae
Clements
and
Choat
(1997); Choat et al. (2002)
Scaridae
Scarus frenatus
Scraper
EAM
Scarus ghobban
Chlorurus sordidus
Scraper
Scraper
EAM
EAM
Scarus schlegeli
Scraper
EAM
Bellwood
and
Choat
(1990), Cvitanovic et al.
(2007)
Cvitanovic et al. (2007)
Choat
et
al.
(2002),
Cvitanovic et al. (2007)
Choat
et
al.
(2002),
Cvitanovic et al. (2007)
Siganidae
Siganus argenteus
Browser
Siganus doliatus
Browser
Macroalgae/filament
ous algae
Macroalgae
Kyphosidae
Kyphosus
bigibbus/cornelii
Kyphosus vaigiensis
82
Paul et al. (1990)
Mantyka and Bellwood
(2007a); Fox et al. (2009)
83
15
13
Table 5.2. Results of two-way PERMANOVA for δ N and δ C for a range of primary sources and consumers collected across three regions (Bundegi,
Mandu and Maud) in reef flat habitat at Ningaloo Reef Australia, including estimated variance component (and % variance). * = lack of homogeneity of
dispersion.
2011
Trophic effects through herbivory at Ningaloo Reef
15
13
84
Table 5.3. Results of two-way PERMANOVA for δ N and δ C for a range of primary sources and consumers collected across 3 habitats at Mandu,
Ningaloo Reef Australia, including estimated variance component (and % variance). * = lack of homogeneity of dispersion.
2011
Trophic effects through herbivory at Ningaloo Reef
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 5.1. Map showing the location of regions sampled at Ningaloo Reef, west coast of
Australia. Three inserts on the left show each of the three regions sampled, with the dotted lines
indicating the boundaries of the reef flat. Bottom insert demonstrates the cross-section structure
of the fringing reef environment, indicating the three habitats: reef slope, reef flat and lagoon.
85
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 5.2. Mean δ15N and δ13C values (± SE, n=3-5) for a range of primary sources and
consumers collected in Reef Flat habitat across three regions (a. Bundegi, b. Mandu and c.
Maud) at Ningaloo Reef, west coast of Australia.
86
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 5.3. Mixing model outputs showing the proportional distribution of contributions of the
main potential sources for the diets of three species of herbivorous invertebrates in the reef flat
habitat in three regions of Ningaloo Reef, west coast of Australia. Lines indicate the 5 to 95
percentiles, grey bars indicate the 25 to 75 percentiles, and vertical line within shaded bar
indicates the median value.
87
Figure 5.4. Mixing model outputs showing the proportional distribution of contributions of the main potential sources for the diets of three species of
Siganidae and three species of in the reef flat habitat in three regions of Ningaloo Reef, west coast of Australia. Lines indicate the 5 to 95 percentiles, grey
bars indicate the 25 to 75 percentiles, and vertical line within shaded bar indicates the median value.
Figure 5.5. Mixing model outputs showing the proportional distribution of contributions of the main potential sources for the diets of five species of
Acantharidae and Kyphosus spp. (Kyphosidae) in the reef flat habitat in three regions of Ningaloo Reef, west coast of Australia. Lines indicate the 5 to 95
percentiles, grey bars indicate the 25 to 75 percentiles, and vertical line within shaded bar indicates the median value.
Trophic effects through herbivory at Ningaloo Reef
2011
15
13
Figure 5.6. Mean δ N and δ C values (± SE, n=3-5) for a range of primary sources and
consumers collected across three habitats (a. Lagoon, b. Reef Flat and c. Outer Reef) at
Mandu, Ningaloo Reef, west coast of Australia
90
Figure 5.7. Mixing model outputs showing the proportional distribution of contributions of the main potential sources for the diets of three species of
invertebrates and two species of Scaridae across three habitats at Mandu, Ningaloo Reef, west coast of Australia. Lines indicate the 5 to 95 percentiles,
grey bars indicate the 25 to 75 percentiles, and vertical line within shaded bar indicates the median value.
.
Figure 5.8. Mixing model outputs showing the proportional distribution of contributions of the main potential sources for the diets of four species of
Acanthuridae and one species of Kyphosidae across three habitats at Mandu, Ningaloo Reef, west coast of Australia. Lines indicate the 5 to 95
percentiles, grey bars indicate the 25 to 75 percentiles, and vertical line within shaded bar indicates the median value.
Trophic effects through herbivory at Ningaloo Reef
2011
Discussion
Variability among roving herbivores
15
13
Using stable isotopes (δ N and δ C) and mixing models, our results suggest that
macroalgae are common components in the diet of a wide range of roving herbivorous fish
species in the coral-reef system at Ningaloo Reef on the eastern boundary of the Indian
Ocean. This complements the results Chapters 2 and 3, which showed a high diversity of
fish species biting assays of Sargassum.
The contribution of macroalgae to the diets of species designated as scrapers, including S.
schlegeli, S. ghobban, A. dussumieri, A. triostegus and C. striatus, in this study is perhaps
surprising given that these species are considered to consume EAM based on their jaw
morphology, gut contents and fatty acids in their guts (Bellwood & Choat 1990, Choat et al.
2002, Choat et al. 2004). Scrapers are generally considered to play a negligible role in the
removal of macroalgae( Hoey & Bellwood 2010b), but have exhibited high bite rates on
Sargassum assays in the GBR and Ningaloo Reef (Fox & Bellwood 2007). At Ningaloo, S.
ghobban and S. schlegeli were shown to contribute 10 and 29% of the bites on algal assays
(Chapter 3), but they suggested that this may be associated with the ingestion of epiphytes
on the macroalgae rather than the thalli itself. We cannot discount this, as we did not include
epiphytes as a potential dietary source in our analyses and it is possible that they could
exhibit similar stable isotope values to some of the macroalgae considered in our study.
However, our observations indicate that epiphytes were not present in large quantities on the
macroalgae collected during the study. Furthermore, Scarus species have been shown to
consume a range of macroalgae, though they exhibit strong preferences for some over
others (Mantyka & Bellwood 2007a). Thus, it is plausible that, while those scraper species
often remove EAM, they may selectively ingest particular components of the EAM, e.g.
microalgae. Alternatively, they may ingest macroalgae and therefore could play a greater
role in the removal of macroalgae in coral-reef systems than previously thought.
Kyphosus vaigiensis and N. unicornis have been shown as major drivers of macroalgal
herbivory in coral reefs in the GBR (Cvitanovic & Bellwood 2009, Hoey & Bellwood 2009,
Bennett & Bellwood 2011) and at Ningaloo Reef (Chapters 3 and 4). Both species display
high levels of macroalgal consumption, based on mass standardised bite rates of
Sargassum assays, at both Ningaloo Reef (24.3% and 18.0%, respectively) and the Keppel
Islands in the GBR (68.24% and 10.86%, respectively). The possession of gut flora that
allows them to assimilate large algae allows them to feed on a range of macroalgae,
including brown, red and green algae (Clements & Choat 1997).
The importance of the browsers Naso unicornis and kyphosids as grazers of macroalgae in
coral-reef systems was further supported in the current study. Naso unicornis and Kyphosus
spp. (K. vaigiensis and K. biggosus) were shown to consistently assimilate large brown
algae, including Lobophora variegata (with or without other brown algae), along with some
foliose red algae. Lobophora variegata ranges in palatability across tropical regions, but
appears to be palatable at Ningaloo Reef (see Chapter 2). In comparison, it was surprising
that Sargassum was not shown to contribute substantially to their diet, since grazing rates by
Kyphosus spp. on this brown alga can be high (Chapters 3 and 4). Also, feeding preference
experiments indicated that Sargassum was consumed far more rapidly than Lobophora
(Vergés, unpubl. data). The reef-flat habitat at Maud and Mandu is dominated by
Sargassum, Hypnea, Turbinaria and Lobophora (Vanderklift, unpubl. data). While
Sargassum is far more dominant in the lagoon habitat (Vergés, unpubl. data), few
herbivorous species appear to occur in the reef-flat habitat (Chapter 3). However, this is
likely to explain the high contribution of Sargassum to the diet of N. unicornis in the lagoon
since it was shown to occur in this habitat. The limited contribution of Sargassum to the diets
of those browsers in the reef-flat and reef-crest habitats in this study is likely to be due to the
low availability of Sargassum and higher fish abundances in those habitats compared to the
lagoon, suggesting a more finite food resource which could lead to other less palatable
93
Trophic effects through herbivory at Ningaloo Reef
2011
algae, such as Lobophora and Turbinaria (Bolser & Hay 1996, Bittick et al. 2010) being
consumed.
Compared to N. unicornis and kyphosids, our results suggest that other browser species,
such as the siganids S. argenteus and S. doliatus and the acanthurid Z. scopas appeared to
rely less on large brown macroalgae and more on foliose red algae or EAM at Ningaloo.
Despite Siganus species being responsible for large numbers of bites on Sargassum assays
in both Ningaloo Reef and Keppel Islands in the GBR, they contribute little to the removal of
this macroalga only in the former region (Vergés et al. Chapter 3, Micheal et al. Chapter 4,
Bennett & Bellwood 2011). Similarly, while Z. scopas was shown to take large numbers of
bites of Sargassum assays, it was responsible for only small amounts of macroalgal removal
at Ningaloo (Vergés et al. Chapter 3, Michael et al. Chapter 4). This is perhaps not
surprising, given that the mixing models showed that all three species consumed a variety of
algal sources at Ningaloo. Indeed, S. doliatus displays a greater preference for Hypnea
(Mantyka & Bellwood 2007a) and perhaps other foliose red algae over brown algae. The
long-term assimilation of material from a range of sources shown through the present study
suggests that large brown macroalgae are not the main dietary source for these species.
Invertebrates consumed a range of macroalgae particularly, with evidence of some EAM
also being consumed. Some degree of consistency was evident across species, particularly
at Mandu where Lobophora/Turbinaria and EAM formed the main part of the diets of both
species of gastropod and the urchin. Turbo spp. feed on a range of macroalgae (Trowbridge
1995, Foster et al. 1999, Wernberg et al. 2008), but some species prefer large brown algae
(Cox & Murray 2006). Similarly, urchins, including Echimometra species, feed on a range of
algae, including red and brown algae (de Loma et al. 2002, Yatsuya & Nakahara 2004).
Despite Turbinaria being considered to be unpalatable (Bittick et al. 2010), it forms a major
part of the diet of the urchin Tripneustes gratilla in a French Polynesian coral-reef system (de
Loma et al. 2002). This species has also been shown to ingest detritus (de Loma et al.
2002). Thus, the consumption of Lobophora and Turbinaria, possibly as early recruits, as
well as EAM, by E. mathaei is highly plausible.
Spatial variability in the importance of food sources to herbivores
Naso unicornis and Kyphosus spp., which are the main browsers at Ningaloo Reef (Vergés
et al. Chapter 3, Michael et al. Chapter 4), almost consistently consumed brown algae
across habitats and regions separated by 100s of metres and 10s of kilometres, respectively.
The predicted contributions of brown algae to N. unicornis and Kyphosus spp. were highly
uniform across regions or habitats, with 5-95 percentiles being similar to the medians for
those sources at Bundegi and Mandu, and in the outer reef, reef flat and lagoon for the latter
13
region. Furthermore, the variance components of particularly δ C for both fish species were
greater at the within-region or within-habitat level, which could partly reflect similar patterns
in the variance components for all brown algae. Thus, stable isotopes of sources and
consumers varied at the level of individuals rather than at broad spatial levels.
Foliose red algae made greater contributions to the diets of a range of browsing species,
including N. unicornis and Kyphosus spp. at Maud where Acanthophora spicifera made
higher contributions to their diets. This does not appear to be related to shifts in food
availability since algal assemblages are similar between Maud and Mandu (Vanderklift,
unpubl. data), but may partly reflect the higher variability in the stable isotope values across
regions for this red alga. In comparison, the contributions of foliose red algae to the diet of
the rabbitfish Siganus doliatus were more consistent across regions, consuming either
Hypnea or A. spicifera. Since this fish species has been shown to strongly select Hypnea on
the GBR (Mantyka & Bellwood 2007a), consistency in its diet is likely to reflect food
preferences. In comparison, the diet of S. argenteus was more variable across regions,
suggesting that this species exhibits lower levels of selection of its food sources like S.
canaliculatus on the GBR (Mantyka & Bellwood 2007a). Assimilated food for S. argenteus
may thus reflect changes in access to its preferred food resources.
94
Trophic effects through herbivory at Ningaloo Reef
2011
Even species that are considered to graze on EAM showed high variability in their food
sources among regions and habitats, though there was also high variation within region and
habitat. For example, the diet of S. sordidus was dominated by EAM at Bundegi, but by
brown algae at Mandu. Since EAM was also a major component of the diet of S. schlegeli at
Bundegi but not Mandu, the dominant food sources may reflect EAM availability.
Invertebrates also generally showed a high degree of variability in stable isotopes among
habitats but not regions, and their food sources varied among regions. Diets of Echinometra
spp. and other urchins have been shown to reflect the abundance of algae and thus
availability (Cobb & Lawrence 2005, Hiratsuka & Uehara 2007), suggesting that the
variability across regions and habitats likely reflects the availability of, or access to, food
resources. As stated earlier, Echinometra spp. have been shown to consume Turbinaria
ornata or other brown algae, suggesting that access to these resources may determine the
type of food assimilated by these grazers.
95
Trophic effects through herbivory at Ningaloo Reef
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CHAPTER 6. The role of herbivory on the spatial distribution of
recruiting and established algal communities in coral versus
algal dominated habitats
Christopher
Doropoulos,
Adriana
Vergés,
David
Abecasis,
Glenn
A.
Hyndes
Introduction
The replacement of corals by fleshy macroalgae is a common effect of large scale reef
degradation on coral reefs due to disturbance, nutrient loading, and reduced trophic function
(Hughes 1994, Bellwood et al. 2004, Sandin et al. 2008). The shift from coral to macroalgal
dominance on coral reefs can occur as a result of disturbance and mortality to the coral
community (Done 1992, Hughes 1994), after which algae can proliferate due to their
competitive advantage in recruitment, rapid growth, and early dominance of newly available
space (Littler & Littler 1999, Diaz-Pulido & McCook 2002, Connell et al. 2004). Interactions
between coral and algae affect the benthic community structure on coral reefs, and are
influenced by direct coral-algal competition (Jompa & McCook 2002, Diaz-Pulido et al. 2009)
and indirectly by herbivory (Sammarco 1980, Lirman 2001, Hughes et al. 2007). The lack of
a diverse herbivore community reduces the resilience of reefs and their ability to recover
following disturbances, which can result in a phase-shift from coral to macroalgal dominated
ecosystems (Hughes 1994, Scheffer et al. 2001, Bellwood et al. 2004, Cheal et al. 2010).
The dramatic phase-shifts from coral to algal dominance observed in some coral reefs
has resulted in the widespread perception of abundant macroalgae as characteristic of
degraded coral reef ecosystems. Two main processes are often proposed to greatly
influence macroalgal abundance in degraded systems: anthropogenic nutrient loading
(bottom-up effects) and overharvesting of herbivore populations (top-down effects)
(Burkepile & Hay 2006). In coral reefs, many studies have investigated the relative
importance of these two processes (e.g. Hatcher & Larkum 1983, Lapointe 1997, Smith et al.
2001, Diaz-Pulido & McCook 2003, McClanahan et al. 2003, Albert et al. 2008, Smith et al.
2010), and a recent meta-analysis of 54 field experiments revealed that herbivores have a
greater effect on algal production and distribution compared to nutrient inputs (Burkepile &
Hay 2006). Herbivores can directly influence algal abundance by consuming both the algal
recruits (Diaz-Pulido & McCook 2003) and adults (Hoey & Bellwood 2010b, Bennett &
Bellwood 2011). They often suppress the growth of turf and upright macroalgae, and their
removal can cause algal proliferation leading to reduced juvenile and adult coral abundance
and growth (Sammarco 1980, Lirman 2001, Jompa & McCook 2002, Box & Mumby 2007,
Hughes et al. 2007, Birrell et al. 2008). Although grazers are a significant component of reef
resilience by mediating coral-algal competition, few studies have examined the effect of
herbivory on both recruiting and adult algal community dynamics.
Coral reefs are complex ecosystems that are characterised by spatial mosaics of
different habitats. Recent studies have challenged the traditional view that intact reefs are
only coral dominated habitats with small algal populations and propose that relatively healthy
coral reef ecosystems may be regarded as spatial mosaics of adjacent coral- and algaldominated benthic communities (Wismer et al. 2009, Johansson et al. 2010, Vroom & Braun
2010). Macroalgal abundance is often related to space availability and coral cover (McCook
et al. 2001, Williams et al. 2001). Algal recruitment experiments demonstrate that high levels
of herbivory can significantly decrease the amount of fleshy algae recruiting to new
substrates (Diaz-Pulido & McCook 2003, McClanahan et al. 2003, Burkepile & Hay 2006),
which is often correlated with increased levels of bare substrata and crustose coralline algae
(CCA) (Hixon & Brostoff 1996, McClanahan 1997, Belliveau & Paul 2002, Albert et al. 2008).
Yet, there is often high spatial and temporal variability associated with algal recruitment and
herbivory in reef ecosystems (Airoldi 2000, Williams et al. 2001, Paddack et al. 2006,
Bonaldo & Bellwood 2011). Indeed, local processes can affect the distribution of both benthic
algae and herbivores, which creates patchiness in reefs (Klumpp & McKinnon 1992, Hixon &
96
Trophic effects through herbivory at Ningaloo Reef
2011
Brostoff 1996, Airoldi 2000). Thus, herbivory is likely to play a different role on algal
recruitment dynamics in adjacent reef habitats by influencing both the recruiting and adult
algal communities.
In this study, we aimed to determine the effect of herbivory and habitat type on the algal
community dynamics of a relatively pristine coral reef ecosystem, Ningaloo Reef (Western
Australia). Monitoring surveys were conducted for nine months to identify seasonal patterns
of the benthic and herbivorous fish communities in lagoon and reef-flat habitats: two distinct
neighbouring areas of coral reefs dominated by algae and coral, respectively, in terms of
percentage cover of substratum (Vergés et al. Chapter 2) . A manipulative experiment was
set up to quantify the effects of habitat and herbivory on algal recruitment over a six month
period, using recruitment tiles and herbivore exclusion cages. We tested the hypotheses
that: (1) algal recruitment would differ between the two habitats; and (2) this would be
influenced by herbivory.
Materials and Methods
Study location
This study was conducted in the Ningaloo Marine Park, Western Australia, a coral reef with
high levels of endemism and diversity (Roberts et al. 2002) that has been managed as a
marine park since 1987, and was rezoned in 2005 to include 33% of the park as sanctuary
(CALM & MPRA 2005). It is found in an arid-zone system that is not subject to any major
anthropogenic threats and is generally considered pristine (Johansson et al. 2010). Ningaloo
Reef is the largest fringing reef in Australia and the only extensive fringing reef on the west
coast of any continent (Collins et al. 2003). It is a narrow fringing reef approximately 270 km
in length that forms a discontinuous barrier adjacent to the North-west Cape, between the
latitudes 21° 47’ E and 24° 00’ S. It consists of 16 major habitat types, differing in relative
dominance of coral, macroalgal, and sand/rubble cover depending on the habitat type within
this large coral reef ecosystem (Cassata & Collins 2008). The outer reef slope and crest are
exposed to high wave energy that forms typical spur and groove morphology to depths of 30
m and support a diverse hard coral assemblage (Cassata & Collins 2008). The reef flat and
lagoon are protected from this wave energy and are influenced by tidal movement which has
a maximum range of ~2m during spring tides. The reef flat is dominated by tabulate
Acropora and the lagoon is dominated by rubble, sand, and patches of fleshy macroalgae
(Cassata & Collins 2008).
We conducted our study from August 2008 until May 2009 in the Mandu sanctuary zone (22°
08’ S, 113° 45’ E) towards the northern section of Ningaloo Reef. Mandu sanctuary is
approximately 8 km long by 2 km wide, and has a lagoon ~600 m wide and a reef flat ~400
m wide. We haphazardly selected two sites within each lagoon and reef-flat habitat, with the
lagoon and reef-flat sites separated by ~400 m, and each site within a habitat separated by
~400 m.
Fish community characterisation
Fish abundance and biomass were characterised for each site in three seasons: spring
(November 2008), summer (February 2009) and autumn (May 2009). A diver swam a 25 m
transect at a constant speed (ca 8 minutes per transect) and counted the abundance and
size class (to 5 cm) of individual nominally herbivorous fish along 2.5 m on both sides of the
transect line. Four replicate transects were conducted at each site at each time period, and
each transect was separated by at least 10 m. Abundance data was converted to biomass
b
using the allometric length-weight conversions W = a * TL , where W is weight in grams, TL
is total length in cm, and parameters a and b are constants obtained from the literature
(Froese & Pauly 2005). Fish surveys were always conducted at least two hours after sunrise
and two hours before sunset. Individuals were recorded from the four major roving herbivore
97
Trophic effects through herbivory at Ningaloo Reef
2011
families to the lowest possible taxa (generally to species, always to genus). We recorded a
total of 20 species from the Acanthuridae, Kyphosidae, Labridae (parrotfish) and Siganidae
families. Initial phase parrotfish were recorded as a single group (Scarus IP).
Benthic community characterisation
2
Macroalgal biomass was measured by clearing three 0.25 m haphazardly placed quadrats
of all macroalgae (thalli larger than 2 cm) at each site-habitat combination. Macroalgal
biomass was sampled in the same three seasons described above for the fish assemblages.
Algal samples were bagged and returned to the laboratory, where they were sorted to genus
level (where possible) and weighed. Algal taxa that we were unable to identify were
classified according to broad functional groups (brown, green or red; filamentous, encrusting
or foliose).
To capture seasonal changes in the benthic community composition in each habitat, the
benthos was surveyed at each site-habitat combination over a year. Surveys took place in
winter (August 2008), spring (November 2008), summer (February 2009) and autumn (May
2009). Benthic communities at each site in the lagoon and reef-flat habitats were sampled
using a line intercept method following Fox and Bellwood (2007). A 10 m transect tape was
laid haphazardly across the reef, and a diver recorded the nature of the substrate directly
touching the tape measure at 1 m interval points and at points 1 m perpendicular to the right
and left of the tape. The substrate categories used were: live coral; CCA; epilithic algal
matrix (EAM; sensu Wilson et al. (2003) which includes dead coral, turf algae, and detritus);
fleshy macroalgae (> 1cm height); and sand. We conducted a total of 6 replicate transects at
each site in the lagoon and reef-flat habitats (total of 30 points per replicate).
Algal recruitment experimental design
To assess the influence of large herbivorous fish on algal recruitment in habitats dominated
by either macroalgae or coral, herbivore exclusion cages were installed at the two sites in
the lagoon and reef flat. The cages were triangular in shape, and measured 50 x 50 x 50 cm.
The corners were marked with rebar, to which PVC coated wire mesh (2.5 x 2.5 cm mesh
size) was attached. In the caged plots, the wire material covered all sides and the top of the
plots. A skirting edge on the sides of the cages was designed to prevent large benthic
invertebrates from accessing the cages. Open (uncaged) plots were marked with the rebar,
and partial cages were used as a control for cage artefacts and consisted of one side and a
roof – to maintain the potential influence of the cage while allowing access to all herbivores.
We installed six plots per treatment per site, and these were scrubbed clean once a month
throughout the experimental period. Some of the plots at the reef-flat sites, from all three
treatments, became inhabited by territorial damselfish. These fish were not removed but
were included in analyses as a covariate due to their influence on benthic algal communities
(see review by Ceccarelli et al. 2001).
Recruitment tiles were installed in the plots in August 2008 and collected in February 2009.
The tiles measured 10 x 10 cm and were made from PVC. Although PVC is not a natural
substrate, studies have demonstrated that algal community composition is not altered by this
material compared to natural materials made from dead corals in long-term algal recruitment
experiments (Hixon & Brostoff 1996). The tiles were sanded to roughen the surfaces, which
created irregular depressions on the tiles to allow for algal attachment (Hixon & Brostoff
1996, Smith et al. 2001, Smith et al. 2010). The recruitment tiles were attached to a piece of
horizontal wire mesh, which was elevated 10 cm from the benthos in each plot. This
standardised the attachment substrate and avoided smothering of the tiles by sediment.
In February 2009, six months after deployment, the two tile replicates from each plot were
carefully removed and placed in individual zip-lock bags in situ. After collection, the tiles
were frozen until laboratory processing. In the laboratory, each tile was thawed and rinsed
with seawater to remove any sediment, and any small invertebrates were removed with
98
Trophic effects through herbivory at Ningaloo Reef
2011
forceps. To determine the percent cover of the algal community on the top surface of each
tile, a point-intercept method was used by placing a transparent plastic sheet with a grid of
25 dots on top of each tile. Using a dissecting microscope, the algae or substrata present
beneath each point were classified into one of the following categories: bare tile, CCA,
encrusting fleshy algae (EFA), turf algae (red, green or brown), and fleshy macroalgae
(which included erect macrophytes such as Dictyota, Laurencia, Lobophora, Padina, and
Sargassum).
Following classification, the algae from the tile was scraped and oven dried at 60° C for ≥ 48
-4
hours, after which the algae were weighed to 10 g. In order to estimate fleshy algae
biomass, we added HCl acid (10%) to the vials to remove any carbonate associated with
sand, coralline algae, and shell fragments. Vials were again placed in the oven at 60° C for ≥
48 hours and reweighed.
Data analyses
Initial inspection of each univariate data set was conducted to check for normal distribution
using the K-S test, and if the data did not conform to a normal distribution after
transformation it was analysed using P values generated from 9999 permutations.
Homogeneity of variance was explored using Cochran’s test for univariate data and with
permDISP for multivariate data. Bray-Curtis similarity was used to create resemblance
matrices for all multivariate data, which were analysed with P values generated from 9999
permutations. When necessary, the data were transformed to meet the assumptions of
homogeneity. When overall significant differences were found (P < 0.05), pair-wise
comparisons were conducted.
Total herbivore and algal biomass were tested using mixed effects analysis of variance
(ANOVA) with habitat (2 levels) and season (3 levels) as fixed factors and site (2 levels)
included as a random factor nested in habitat. The biomass of the fish and algal communities
were used to assess patterns in their composition using mixed effects multivariate analysis
of variance (PERMANOVA) with the same model described above. To investigate benthic
community cover we used the same mixed effects PERMANOVA model, but with 4 levels for
season. We then analysed the percent cover of each benthic group using permutational
ANOVA with the model described. Principal coordinate analysis (PCO) was conducted to
visualise similarities between the fish, algal, and benthic communities amongst habitats
using Bray-Curtis distance as our metric. We investigated the taxa that contributed most
strongly to the dissimilarities from the different habitats by using correlations > 0.6 based on
Spearman ranking.
The mean percent cover on the top surface of the tiles was used to analyse the composition
of the recruiting algal community using a mixed effects multivariate analysis of covariance
(PERMANCOVA). Habitat (2 levels) and herbivory (3 levels) were included as fixed factors,
with site (2 levels) included as a random factor nested in habitat. Damselfish were included
as the covariate because they established territories in some of the experimental plots in the
reef-flat sites. Following this, the mean percent cover of each individual functional group and
the total fleshy algal biomass were analysed using permutational ANCOVA with the same
model.
All ANOVAs were performed using GMAV5 (coded by A. J. Underwood and M. G. Chapman,
University of Sydney, Australia). Permutation, covariate, and multivariate analyses were
performed using Primer-E v6 (Clarke & Gorley 2006) with the PERMANOVA+ add-on
package (Anderson et al. 2008b).
99
Trophic effects through herbivory at Ningaloo Reef
2011
Results
Roving herbivorous fish biomass and community composition
Significant differences in the total biomass of roving herbivorous fish were found between the
lagoon and reef-flat habitats, where the biomass in the reef-flat was at least an order of
magnitude higher than the lagoon (Table 6.1a; Fig. 6.1a). In contrast, we detected no
significant differences in total herbivorous fish biomass between seasons or sites (Table
6.1a). The herbivorous fish community composition also differed significantly between the
lagoon and reef-flat habitats, and there were no clear seasonal patterns in fish community
composition (Table 6.1b). The PCO of the herbivorous fish community composition yielded
two components that explained 58.1 % of the variance (Eigenvalues 31923 for PCO1 and
21084 for PCO2, Figure 6.2a). Although there was some overlap, the separation between
the lagoon and reef-flat habitats were clustered predominantly on the PCO1 axis. Spearman
ranking identified Acanthurus triostegus, Chlororus sordidus, Scarus rivulatus, S. schlegeli
and Siganus argenteus as the dominant species correlated with the reef flat, while
Acanthurus sp. and Scarus IP were associated with both habitats. The samples from within
both the lagoon and reef-flat habitats were spread across the PCO2 axis indicating variability
in the herbivorous fish community across sites within habitats (Table 6.1b, Fig. 6.2a).
Benthic algal biomass and community composition
The total algal biomass was highly seasonal and it was greatest in the lagoon compared to
the reef flat during autumn only (significant Habitat x Season interaction, Table 6.2a). Both
reef habitats displayed a seasonal increase in algal biomass from spring and summer to
autumn, but this was only significant in the lagoon where the change in fleshy algae biomass
2
increased from ~40 to 187 g per 0.25m (Fig 6.1b). In contrast, the benthic algal community
composition displayed significant differences amongst the lagoon and reef-flat habitats, but
this was not affected by seasonality (Table 6.2b). Although there was large site within habitat
variability, there was some separation between the habitats on the PCO1 axis that explained
40.3 % of total variation (Eigenvalue 26189; Table 6.2b; Fig. 6.2b). The lagoon was
characterised by Sargassum, whilst Lobophora variegata and Hypnea characterised the reef
flat (Fig. 6.2b).
Benthic community cover
Multivariate analysis of the benthic community cover revealed a strong distinction between
the lagoon and reef-flat habitats (Table 6.3). The community composition was separated on
the PCO1 axis, which explained 89.4 % of the total variation (Eigenvalue 57638, Fig. 6.3).
Spearman correlations indicated that differences between habitats were clearly driven by
fleshy macroalgae and sand in the lagoon, and by live coral and EAM in the reef flat (Fig.
6.3). The lagoon had an average of ~27 % macroalgae cover (Fig. 6.4b), which was
significantly higher than the reef flat in summer and autumn but not in winter and spring
(significant Habitat x Season interaction, Table 6.4). The reef flat had an average of ~47 %
live coral cover and ~27 % EAM cover (Fig. 6.4a and 6.4c), which were both significantly
greater than in the lagoon (Table 6.4). Although CCA was generally higher in the reef flat
compared to the lagoon (Fig. 6.4c), this was non-significant (Table 6.4).
Recruitment tile algal community composition and biomass
The community composition of the algae found on the recruitment tiles was strongly affected
by the experimental herbivory treatments. Irrespective of habitat, algal community
composition was significantly different between tiles excluded from herbivores (closed
100
Trophic effects through herbivory at Ningaloo Reef
2011
treatments) compared to those exposed to herbivores (open and partial treatments; post-hoc
analysis Closed ≠ Partial = Open, Table 6.5). These differences were characterised by
significantly higher cover of bare tile and fleshy macroalgae in the herbivore exclusion
treatment, in both the lagoon and reef-flat habitats (Fig. 6.5a & 6.5e; Table 6.6). The
presence of damselfish significantly affected the community composition of the algae present
on the recruitment tiles (Table 6.5). Damselfish territories were only established at plots in
the reef-flat sites, where their presence significantly increased turf cover compared to the
lagoon habitat, irrespective of herbivore exclusion treatment (Fig. 6.5d; Table 6.6).
Damselfish presence also tended to decrease CCA cover on the recruitment tiles in the reefflat sites (Fig. 6.5c) but this effect only approached statistical significance (p = 0.079; Table
S6). Within each habitat there was significant variability amongst sites in the multivariate
analysis (Table 6.5) and for the CCA and turf cover in the univariate analyses (Table 6.6).
Following a similar pattern to the algal cover on the recruitment tiles, the biomass of fleshy
algae was significantly higher on tiles from the herbivore exclusion treatments compared to
those exposed to herbivores in both the lagoon and reef-flat habitats (Fig. 6.6; Table 6.7).
-2
Total algal biomass in the lagoon (~7 g 100 cm ) was slightly higher than the reef flat (~4 g
-2
100 cm ), but this difference was not significant, probably due to the high variability between
sites within each habitat (Table 6.7).
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Trophic effects through herbivory at Ningaloo Reef
2011
Table 6.1. Mixed effect (a) permutational ANOVA assessing total biomass and (b)
PERMANOVA assessing community composition of herbivorous fish between habitats and
seasons. Total biomass data were square root transformed and community composition was log
(x + 1) transformed prior to analysis. Relevant significant probabilities are indicated in bold.
Source
of
df
MS
variation
A. Total biomass
Habitat
1
18528.0
Season
2
2993.0
Site (Ha)
2
142.3
Ha x Se
2
2372.1
Se x Si (Ha)
4
1213.8
Residual
36
1496.6
B. Community composition
Habitat
1
20601.0
Season
2
1855.1
Site (Ha)
2
4459.2
PsuedoF
P
(perm)
Conclusion – Pair-wise
130.18
2.47
0.09
1.95
0.81
0.0083
0.2000
0.9431
0.2578
0.5944
Lagoon < Reef flat
4.61
0.97
3.38
0.0269
0.4759
0.0003
Lagoon ≠ Reef flat
Ha x Se
Se x Si (Ha)
Residual
0.76
1.45
0.6153
0.109
2
4
36
1456.0
1907.1
1318.1
102
39717.2
27061.0
4869.2
17478.7
1
2
2
2
4
26
24
Se x Si (Ha)
Pooled
Residual
2103.5
1739.6
1192.4
Ha x Se
Se x Si (Ha)
Residual
2
4
24
11825.0
4258.6
2417.5
B. Community composition
Habitat
1
Season
2
Site (Ha)
2
2031.8
3558.0
3448.8
MS
df
Source
of
variation
A.
Total
biomass
Habitat
Season
Site (Ha)
Ha x Se
1.21
1.46
0.3427
0.1081
0.0199
0.1062
0.0366
P
(perm)
PsuedoF
4.89
2.45
2.03
0.6860
0.0025
0.0170
0.2722
0.0356
P
0.57
11.16
13.32
1.37
8.60
F
103
Lagoon ≠ Reef flat
Conclusion: Pair-wise
Summer and Spring: Lag = RF; Autumn:
Lag > RF
Lagoon: Summer = Spring < Autumn;
Reef flat: NS
Conclusion: SNK
Table 6.2. Mixed effects (a) ANOVA assessing total biomass and (b) PERMANOVA assessing community
composition of benthic algae between habitats and seasons. Community composition data were square root
transformed prior to analysis. Relevant significant probabilities are indicated in bold. Pooling was in accordance
with Underwood (1997).
2011
Trophic effects through herbivory at Ningaloo Reef
45734.0
480.0
535.6
673.0
228.6
1
3
2
3
6
80
Residual
111.0
MS
df
of
Source
variation
Habitat
Season
Site (Ha)
Ha x Se
Se x Si (Ha)
PsuedoF
85.38
2.10
4.82
2.94
2.06
P
(perm)
0.0043
0.1001
0.0054
0.0811
0.0507
104
Lagoon ≠ Reef flat
Conclusion: Pair-wise
Table 6.3. Results of the mixed effects PERMANOVA assessing benthic community cover between habitats and
seasons. Data were sin (√x) transformed prior to analysis. Relevant significant probabilities are indicated in bold.
2011
Trophic effects through herbivory at Ningaloo Reef
3
2
3
6
8
0
Season
Site (Ha)
Ha x Se
Se x Si (Ha)
Residual
1.50
1.67
1.60
0.31
293.97
0.19
0.27
0.2031
0.8227
0.0024
Live coral
Pseud
P
o-F
(perm)
1.14
2.28
1.18
3.50
9.24
Pseud
o-F
CCA
0.182
9
0.343
0.095
6
0.095
4
0.316
8
P
(perm)
2.61
1.33
1.61
4.18
180.02
Pseud
o-F
0.0228
0.3503
0.204
0.0571
0.0061
EAM
P
(perm)
2.67
5.16
6.527
2.53
25.11
0.0206
0.0405
0.0024
0.1585
0.038
Fleshy macroalgae
Pseud
P
o-F
(perm)
0.95
1.51
4.26
3.80
78.98
Pseud
o-F
0.4777
0.3063
0.0172
0.078
0.0126
Sand
P
(perm)
105
Live coral – Habitat: Lagoon < Reef flat
EAM – Se x Si (Ha): Lagoon (1) Wi = Sp = Su > Au; Lagoon (2) Sp > Au, all other NS; Reef flat (1) Su > Wi = Au = Sp, Reef flat (2) all NS
Macroalgae – Ha x Se: Season: Winter and Spring NS; Summer and Autumn Lagoon > Reef flat
Habitat: Lagoon Summer < Autumn; all other NS; Reef flat Winter = Autumn > Summer, all other NS
Sand – Habitat: Lagoon > Reef flat
Pair-wise tests for significant effects:
1
Source of
variation
Habitat
df
Table S4. Results of the mixed effects permutational ANOVAs assessing live coral, crustose coralline algae (CCA), epilithic algal matrix (EAM), fleshy
macroalgae, and sand cover between habitats and seasons. Data were sin (√x) transformed prior to analysis. Relevant significant probabilities are indicated in
bold. Winter = Wi; Spring = Sp; Summer = Su; Autumn = Au.
2011
Trophic effects through herbivory at Ningaloo Reef
17433.0
5141.1
4262.9
3185.3
1
1
2
2
6
59
Pooled
Residual
967.8
543.5
MS
df
of
Source
variation
Damselfish
Habitat
Herbivory
Site (Ha)
PseudoF
8.12
2.59
4.17
5.61
P
(perm)
0.011
0.313
0.044
0.002
106
Closed ≠ Partial = Open
Conclusion: Pair-wise
Table 6.5. Results of the mixed effects PERMANCOVA assessing community cover on the recruitment tiles
between habitats and herbivore exclusion. Damselfish presence/absence was included as a covariate. Data
were sin (√x) transformed prior to analysis. Pooling was in accordance with Underwood (1997). Relevant
significant probabilities are indicated in bold.
2011
Trophic effects through herbivory at Ningaloo Reef
1
2
2
2
2
4
1
Habitat
Herbivory
Site (Ha)
Da x He
Ha x He
He x Si (Ha)
Da x He x Si
(Ha)
Residual
1.49
0.51
0.03
0.84
2.07
18.18
2.62
0.01
0.2168
0.7263
0.9757
0.4663
0.1357
0.0116
0.1798
0.9184
Bare tile
PseudoP
F
(perm)
0.06
0.21
2.78
4.87
1.28
1.03
0.01
2.77
PseudoF
P
(perm)
0.8212
0.9269
0.1638
0.0383
0.2908
0.4332
0.9196
0.136
EFA
1.85
4.39
0.07
0.50
9.51
0.33
2.68
4.18
PseudoF
P
(perm)
0.1851
0.004
0.9379
0.6253
0.0004
0.7482
0.2245
0.0799
CCA
3.04
1.29
0.38
0.91
5.82
2.83
2.31
17.30
PseudoF
P
(perm)
0.0872
0.2814
0.7136
0.4484
0.0052
0.1693
0.2325
0.0011
Turf
107
Pair-wise tests for significant effects:
Tile – Herbivory: C > O = P
EFA – Da x He: all NS
CCA – He x Si (Ha): Lagoon (1) C ≠ P = O; Lagoon (2) all NS; Reef flat (1) all NS; Reef flat (2) C = P, C ≠ O, P = O
MA – Herbivory: C > O = P
56
1
Source of
variation
Damselfish
df
0.04
0.33
0.77
1.82
1.72
57.13
1.40
1.60
Pseudo
-F
P
(perm)
0.8277
0.8513
0.5225
0.2221
0.1908
0.0031
0.3046
0.2511
MA
Table 6.6. Results of the mixed effects permutational ANOVAs assessing bare tile, encrusting fleshy algae (EFA), crustose coralline algae (CCA), turf algae, and
fleshy macroalgae (MA) cover on the top side of recruitment tiles between habitats and herbivory. Damselfish presence/absence was included as a covariate.
Data were sin (√x) transformed prior to analysis. Relevant significant probabilities are indicated in bold.
2011
Trophic effects through herbivory at Ningaloo Reef
Trophic effects through herbivory at Ningaloo Reef
2011
Table 6.7. Results of the mixed effect ANCOVA of fleshy algal biomass on the recruitment tiles
between habitats and herbivore exclusion treatments. Damselfish presence/absence was
included as a covariate. Data were log (x+1) transformed prior to analysis. Pooling was in
accordance with Underwood (1997). Relevant significant probabilities are indicated in bold.
Source
of
variation
Damselfish
Habitat
Herbivory
Site (Ha)
Pooled
df
MS
1
1
2
2
65
1.52
1.46
0.77
1.24
0.09
PseudoF
1.94
2.05
8.13
13.29
P
(perm)
0.2044
0.2723
0.0005
0.0001
Conclusion: pair-wise
Closed > Partial = Open
2
Figure 6.1. Mean biomass (± SE) of (a) herbivorous fish per 125 m and (b) fleshy algae per
2
0.25 m in the lagoon and reef-flat habitats during spring, summer and autumn, at Ningaloo
Reef. Note the logarithmic scale on the y axis for herbivorous fish biomass.
108
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 6.2. Principal coordinate analysis comparing (a) herbivorous fish and (b) benthic
algae community composition from lagoon and reef-flat habitats at Ningaloo Reef.
Herbivorous fish data were log (x+1) transformed and benthic algae data were fourth root
transformed prior to analysis. Upward facing solid triangles = lagoon; downward facing
triangles hollow triangles = reef flat. Vector overlays represent correlations > 0.6 based on
Spearman ranking.
109
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 6.3. Principal coordinate analysis comparing benthic community composition (% cover)
from lagoon and reef-flat habitats. Data were sin (√x) transformed prior to analysis. Upward
facing solid triangles = lagoon; downward facing triangles hollow triangles = reef flat. CCA =
crustose coralline algae. EAM = Epilithic algal matrix. Vector overlays represent correlations >
0.6 based on Spearman ranking.
110
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 6.4. Mean cover (% ± SE) of (a) live coral, (b) sand, (c) crustose coralline algae, (d)
epilithic algal matrix, and (e) fleshy macroalgae in lagoon and reef-flat habitats during winter,
spring, summer, and autumn, at Ningaloo Reef. Note the different scales on the y axes for each
functional group.
111
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 6.5. Mean cover (% ± SE) of different functional groups after six months on the
topside of recruitment tiles in lagoon and reef-flat habitats depending on herbivore exclusion
treatments. Note the different scales on the y axes for each functional group. Closed =
herbivores excluded; Partial cage = cage with lid and one side only; Open = no cage.
112
Trophic effects through herbivory at Ningaloo Reef
2011
Figure 6.6. Mean biomass (mg ± SE per 100 cm2) of fleshy algae on recruitment tiles after
six months in lagoon and reef-flat habitats depending on herbivore exclusion treatments.
Closed = herbivores excluded; Partial cage = cage with lid and one side only; Open = no
cage.
113
Trophic effects through herbivory at Ningaloo Reef
2011
Discussion
Distinct patterns in the benthic algal and herbivorous fish communities were observed
between adjacent lagoon and reef-flat habitats at Ningaloo Reef over a full seasonal cycle.
Ningaloo Reef is a relatively pristine fringing coral-reef ecosystem that is dominated by high
coral cover and abundant herbivorous fish communities, and algal recruitment dynamics
were strongly influenced by herbivory in both lagoon and reef-flat habitats. In both habitats,
herbivory equally limited the recruiting algal community, but differences between algal
functional groups were influenced at a localised scale. At a broader spatial scale, large
fleshy macroalgae were seasonally more abundant in the lagoon where the herbivorous fish
community was characterised by fewer and smaller individuals. On the contrary, high coral
cover dominated the reef flat, providing structure for an abundant and diverse herbivorous
fish community that limited fleshy macroalgal abundance via herbivory and reduced
settlement space for algal recruits(Williams et al. 2001, Bejarano et al. 2011, Vergés et al.
Chapter 2).
Patterns in the spatial distribution of benthic coral-reef algae are determined by different
levels of grazing, nutrient inputs, substrate availability, hydrodynamics, and recruitment
(Lubchenco & Gaines 1981, Santelices 1990). In our study, we have clearly demonstrated
that patterns of the spatial distribution of recruiting fleshy algae were influenced by patterns
of herbivory and space availability. When recruitment space was equal in the lagoon and
reef-flat habitats, herbivory was an important mechanism that influenced the abundance of
fleshy macroalgae on the recruitment tiles. Both the cover and biomass of fleshy macroalgae
were significantly higher in the herbivore exclusion treatments, yet there were no differences
in the biomass of the algae between the two habitats. Similarly, recent evidence also
demonstrated that the biomass of adult Lobophora variegata was the same in the lagoon
and reef-flat habitats when herbivores were excluded for 6 weeks from the algae at Ningaloo
Reef (Vergés et al. Chapter 2). This suggests that both the input of algal propagules and
algal productivity in both habitats were similar during the time of our study.
In contrast to the biomass of the caged recruiting fleshy macroalgae in this study, there was
no difference in the algal cover or biomass between the lagoon and reef-flat habitats when
herbivores were allowed access to graze the recruitment tiles. The abundance of herbivores
was relatively low in the lagoon compared to the reef flat; yet, the herbivore community found
in the lagoon habitat nevertheless strongly influenced the recruiting macroalgal community.
Therefore, we suggest that there were no recruitment or nutrient limitations on early algal
cover and growth when space availability was unlimited. Similar to other recruitment studies
that have identified the roving herbivores in limiting the abundance of fleshy macroalgae
(Belliveau & Paul 2002, Diaz-Pulido & McCook 2003, Burkepile & Hay 2006, Albert et al.
2008), herbivory was an important post recruitment process driving early algal community
structure in this coral-reef ecosystem.
While roving herbivorous fish affected the cover and biomass of fleshy macroalgae, apparent
differences in the community composition of the algal recruits between the lagoon and reef
flat was influenced by the presence of territorial damselfish at a local scale. In our study,
damselfish were present at some of the treatments within the reef-flat sites where they
tended to increase the amount of turf and decrease the amount of CCA cover. They were
never found in the lagoon where turf algal cover was relatively low and CCA cover was
relatively high, and they did not affect the amount of fleshy macroalgae cover or biomass in
either habitat. Other studies have demonstrated that large roving herbivores have a
significant influence on the broad benthic algal community structure (Mumby et al. 2006,
Paddack et al. 2006, Fox & Bellwood 2007, Wismer et al. 2009, Vergés et al. Chapter 2),
while the farming practices of territorial damselfish can instead influence turf productivity and
diversity at smaller scales (Klumpp et al. 1987, Ceccarelli et al. 2005, Gobler et al. 2006,
Hoey & Bellwood 2010c). Territorial damselfish often chase away larger roving herbivores to
protect the turf algal communities that they farm, influencing patterns of algal recruitment and
114
Trophic effects through herbivory at Ningaloo Reef
2011
succession (Hixon & Brostoff 1996, Ceccarelli et al. 2001, Ceccarelli et al. 2006). Although
we did not quantify their effect on the standing algal community, the presence of territorial
damselfish in the reef-flat sites presumably influenced the high EAM cover in the reef flat.
These patches add to the spatial heterogeneity of algae in a coral-dominated reef habitats,
providing an important source of nutrition to meso- and macro-herbivores (Smith et al. 2001,
Wilson et al. 2003).
Distinct differences between the benthic algal communities were observed in the lagoon and
reef-flat habitats, yet these differences were not reflected in the algal biomass and cover
which were seasonally dependent. The lagoon and reef flat had a similar cover and biomass
of fleshy macroalgae through the year, except for autumn when the lagoon was
characterised by a dramatic increase in macroalgae. Similar to studies from the Great Barrier
Reef (GBR) that found seasonal increases in algal cover in autumn (Schaffelke & Klumpp
1997, Lefevre & Bellwood 2010), the increase of macroalgae found in the lagoon at Ningaloo
Reef was dominated by Sargassum. Although we do not exclude the influence of nutrients or
phenology on algal abundance, the difference in macroalgal cover between the lagoon and
reef-flat habitats at Ningaloo Reef appears to be highly influenced by the characteristics of
the herbivorous fish community in both habitats.
In the present study, while the herbivore community found in the lagoon habitat influenced
the recruiting fleshy algal abundance, it was obviously not able to limit seasonal increases in
Sargassum. While we did not directly test whether herbivory was controlling the benthic algal
abundance in our study, it was recently demonstrated that both Sargassum myriocystum and
Lobophora variegata were heavily consumed by grazers in the reef-flat habitat at Ningaloo
Reef, but that herbivores did not consume these algae in the lagoon due to the lack of large
browsers (Vergés et al. Chapter 2). Although herbivory is not uniform, roving herbivorous fish
are known to significantly limit the growth and density of both Sargassum and Lobophora
recruits (Diaz-Pulido & McCook 2003). Thus, we suggest that the algae may have a certain
size, abundance, or seasonal increase in productivity where they are able to escape
consumption by the grazers found in the lagoon at Ningaloo Reef due to their low biomass in
that habitat. On the contrary, the abundance and diversity of the grazer community found in
the reef-flat habitat appears enough to limit macroalgal growth throughout the year. Although
their biomasses were low, Lobophora variegata and Dictyota sp. were characteristic the
macroalgae found on the reef flat. Compared to Sargassum, these macroalgae are not
highly palatable to large herbivores (Steinberg & Paul 1990, Mumby et al. 2005, Bittick et al.
2010), and this may explain their dominance where herbivory is high. Thus, we find that
herbivore abundance and diversity influenced both the abundance and community
composition of macroalgae found in the lagoon and reef-flat habitats.
In general, many studies have demonstrated that tropical algal distribution is highly
influenced by different levels of herbivory (Burkepile & Hay 2006) from echinoids in
Caribbean reefs (Sammarco 1982, Edmunds & Carpenter 2001) to both roving and territorial
fish in most reefs (Klumpp & McKinnon 1992, McClanahan 1997, Ceccarelli et al. 2001,
Bellwood et al. 2004, Hughes et al. 2007, Wismer et al. 2009, Vergés et al. Chapter 2).
There is an abundant and diverse community of herbivorous fish at Ningaloo Reef
dominated by fish from each major functional group (Green & Bellwood 2009), similar to
abundances found in offshore reefs on the GBR (Wismer et al. 2009). Diverse guilds of
herbivores are necessary for resilience on coral reefs (Bellwood et al. 2004, Cheal et al.
2010, Hughes et al. 2010), and their abundance and diversity at Ningaloo Reef suggests that
they support the resilience of this unique fringing coral-reef ecosystem by limiting algal
recruitment. While there were no seasonal changes to the fish community, the abundance of
palatable macroalgae in lagoon habitats at Ningaloo Reef did alter seasonally, yet this was
not found in adjacent reef-flat habitats characterised by live coral. Coral reefs are spatial
mosaics of different habitats, which host a diversity of flora and fauna, and the fish and algal
communities at Ningaloo Reef offer complexity to distinct habitats within small spatial scales
that connect this extensive fringing coral reef.
115
Trophic effects through herbivory at Ningaloo Reef
2011
CHAPTER 7. Main findings and outcomes
The consumption of macrophytes by herbivores is a particularly important ecological process
in coral reefs that supports intricate food webs and strongly contributes to the resilience of
these systems following disturbances such as cyclones or bleaching events (Bellwood et al.
2004; Mumby et al. 2006). Broadly, this project aimed to characterise plant-herbivore
interactions in the Ningaloo Reef Marine Park, and has provided the first quantification of the
process of herbivory in this region. In doing so, it has provided key information to answer the
management questions identified in the Ningaloo Research Program Research Priorities.
The questions relevant to this study are:
1.
2.
3.
4.
What is the species diversity of key flora and fauna in selected representative habitats?
What is the abundance, size composition and distribution of these key species?
Do variations in predator abundance indirectly affect reef ecosystem structure including
the abundance of grazers, algae and corals and corallivorous gastropods (Drupella)?
Are current management arrangements/regulations appropriate for preserving the
biodiversity represented within the park?
The following provides the key findings and relevant management implications under the
headings of the management questions.
What is the species diversity of key flora and fauna in selected representative
habitats, and what is the abundance, size composition and distribution of these key
species?
The biomass and species composition of roving herbivorous fishes varied among the five
regions examined (Bundegi, Mandu, Point Cloates, Maud and Gnaraloo) using underwater
visual censuses. Using underwater video cameras, 23 different fish species were observed
consuming macroalgae, but seven species (Naso unicornis, Kyphosus sp., K. vaigiensis,
Siganus doliatus, Scarus ghobban, S. schlegeli and initial-phase Scarus sp.) together
accounted for 95% of the observed bites across five regions. Of these species, three taxa
were identified as the most important in consuming macroalgal (Sargassum myriocystum)
assays: N. unicornis, Kyphosus sp. and K. vaigiensis. These results were supported by
stable isotope analyses that incorporate nutrients from food sources over far longer periods
than those examined using the assay approach.
The structurally complex coral-dominated outer reef and reef-flat habitats were characterised
by the highest biomass of herbivorous fish and greatest levels of herbivory (based on
Sargassum assays), compared to lagoon habitat. There was also a high degree of variability
in grazing rates among regions separated by hundred of kilometers in the marine park, with
different species responsible for macroalgal removal among those regions.
A transcontinental comparison between Keppel Islands in the Great Barrier Reef (GBR) on
the east coast of Australia and Ningaloo Reef, showed differences in the diversity of the
species observed feeding and the species composition of roving herbivorous fishes between
the two continental regions. In Ningaloo Reef, 23 species were observed biting on
Sargassum, compared with just 8 in the Keppel Islands. The larger number of species
feeding on macroalgae in Ningaloo Reef suggests that there may be higher functional
redundancy among macroalgal consumers in this system, however, a large proportion of
herbivory was dependent on only a few species in both regions.
116
Trophic effects through herbivory at Ningaloo Reef
2011
Management implications and recommendations:
•
•
•
•
We have provided a baseline survey of all nominally herbivorous fish species (fish
density and quantitative feeding activity data) across a range of regions and we have
identified key species that should be closely monitored.
We provide evidence that structural complexity is a key factor influencing herbivory.
Conservation efforts should thus focus on conserving this trait (e.g. protect coral
habitats from anchoring damage).
Although herbivorous fishes are not presently targeted by fishers in NMP, this trophic
group is increasingly being targeted for exploitation elsewhere. We have provided
quantitative data that can be used to support potential management plans aimed at
protecting herbivorous fishes from exploitation on the basis of their critical role for
promoting coral-reef resilience.
A direct comparison of herbivory between different coral-reef systems, indicates that
Ningaloo Reef is a comparatively pristine system.
Do variations in predator abundance indirectly affect reef ecosystem structure
including the abundance of grazers, algae and corals and corallivorous gastropods
(Drupella)?
Collectively, N. unicornis, Kyphosus sp. and K. vaigiensis accounted for only 1-7% of roving
herbivorous fish biomass across regions, yet were responsible for 85-99% of the bite rates
on Sargassum being quantified in these regions (except in Mandu, where browsers were
depauperate). Rates of herbivory could not be predicted based on the distribution of
macroalgal browsers in each region.
There were high levels of variability in the importance of different food sources across both
habitats and regions for some consumers (e.g. Siganus spp.), but consistency for other
species (e.g. Naso unicornis, Kyphosus spp.), which is likely to reflect shifts in food source
availability or feeding preferences.
Herbivory had an equally strong effect on the community composition of algal recruits in the
lagoon and reef-flat habitats, despite the reef flat hosting a herbivorous fish community that
is an order of magnitude greater in terms of biomass than the lagoon, which is characterised
by younger and smaller fish (e.g. Scarus initial phase). Differences among habitats in algal
biomass were strongly influenced by season. Lagoon habitats only have higher biomass
than reef flat habitats during part of the year (late summer/ early autumn). Apparent habitat
differences in community composition of algal recruits were in fact driven by the presence of
damselfish, which were only present in some of the treatments within the reef-flat (but never
in the lagoon).
Management implications and recommendations:
•
•
Variability in grazing rates across NMP, and the species responsible for grazing,
indicates that any future monitoring of key species needs to take place over different
regions of the marine park.
Herbivory is a dominant mechanism that influences the abundance of fleshy
macroalgae when recruitment space is equal in the lagoon and reef-flat habitats. Zoning
needs to consider the movement of key herbivores across habitats when determining
boundaries of management zones.
117
Trophic effects through herbivory at Ningaloo Reef
2011
Are current management arrangements/regulations appropriate for preserving the
biodiversity represented within the park?
Other Node 3.2.2 sub-projects showed no differences in algae or corals between Sanctuary
and Recreational zones, suggesting no apparent cascading trophic effects on benthic
organisms in NMP. Interestingly, those studies also showed higher grazer biomass in
sanctuary zones, particularly in relation to parrotfishes. Our results indicate that total grazer
biomass as a poor indicator of potential grazing, and its use in implying trophic effects could
lead to erroneous conclusions. Only a few of the 23 roving herbivorous species play a
significant role in macroalgal removal.
Other Node 3.2.2 sub-projects have shown that the herbivore Kyphosus sydneyanus moves
between lagoon and reef-slope habitats, but are likely to remain within a region. Our study
has shown the importance of kyphosid species in removing macroalgae, but there is
variability in their abundances across regions. Furthermore, N. unicornis is an important
species for the removal of macroalgae. While these herbivore species are not targeted by
fishers, they are likely to be susceptible to changes in the abundances of higher order
predators such as sharks, which have been shown in other Node 3.2.2 sub-projects to be
affected by fishing activities leading to potential trophic cascades. Furthermore, these
grazers are also likely to be susceptible to non-fishing human disturbances (e.g. oil spills and
coral damage) and natural disturbances that affect coral structure (e.g. cyclones).
Management implications and recommendations:
•
•
•
•
Monitoring the biomass of Naso unicornis, Kyphosus spp. inside and outside sanctuary
zones will provide crucial information of the potential influence of zoning on macroalgal
removal in the NMP, as well as a region’s ability to recover from disturbances that
enhance macroalgal production.
Quantitative data on rates of herbivory from our studies can be incorporated into broadscale fish density data from other projects to model the effects of disturbances and
changes in management strategies on herbivory, and potential effects to the system as
a whole.
Data on the spatial patterns of movement are needed for all key macroalgal grazers to
ensure that sanctuary zones preserve their abundances.
Research is needed to further investigate the potential for indirect ecological effects and
trophic cascades through the removal of higher order predators (e.g. sharks) in the
NMP.
118
Trophic effects through herbivory at Ningaloo Reef
2011
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Publications and Presentations from this Research
Publications
Vergés, A., Vanderklift, M.A., Doropoulos, C. and Hyndes, G.A. (2011). Spatial patterns in
herbivory on a coral reef are influenced by structural complexity but not by algal traits.
PLoS One 6: 1-12.
Vergés, A., Bennett, S. and Bellwood, D. (In review). Herbivore diversity on coral reefs: a
transcontinental comparison. Marine Ecology Progress Series.
Michael, P. (2009). Regional differences in the piscine drivers of macroalgal herbivory in a
coral-reef marine park. Honours Thesis. Edith Cowan University.
Presentations
Michael, P. (2009). Regional differences in the piscine drivers of macroalgal herbivory in a
coral-reef marine park. Honours Seminar. Edith Cowan University.
Vergés, A., Hyndes, G., Vanderklift, M., Babcock, R., Haywood, M., Doropoulos, C., Thomson,
D. (2008). Spatial variation in herbivory on Ningaloo Reef. 2nd Annual Ningaloo
Symposium. 2008.
Vergés, A. Hyndes, G.A. and Vanderklift, M. (2009).Within-habitat variation in fish herbivory in
Ningaloo Reef, Western Australia Indo-Pacific Fish Conference. 2009.
Vergés, A., Michael, P., Hyndes, G.A. and Vanderklift, M. (2009). Magnitude and patterns of
herbivory in Ningaloo Reef. 3rd Annual Ningaloo Symposium. 2009.
Vergés, A. Hyndes, G.A. and Vanderklift, M. (2010). Spatial heterogeneity drivers of macroalgal
herbivory in coral reefs. AMSA National Conference, Wollongong. 2010.
129